Make ItemManager not a singleton (#589)

There is no benefit of it being a singleton
and singletons often bring a bunch of problems
and lead to suboptimal architecture.

Author: leolost2605

src/BaseItem.vala

@@ -312,7 +312,7 @@ public class Dock.BaseItem : Gtk.Box {
      * @param y pointer y position
      */
     public void calculate_dnd_move (BaseItem source, double x, double y) {
-        var launcher_manager = ItemManager.get_default ();
+        var launcher_manager = (ItemManager) get_ancestor (typeof (ItemManager));
         launcher_manager.move_launcher_after (source, (int) index);
     }
 

src/DBus/ShellKeyGrabber.vala

@@ -109,7 +109,16 @@ public interface ShellKeyGrabber : GLib.Object {
             return;
         }
 
-        Dock.ItemManager.get_default ().launch (saved_action_ids[action]);
+        var index = saved_action_ids[action] - 1;
+        var apps = Dock.AppSystem.get_default ().apps;
+
+        if (index < 0 || index >= apps.get_n_items ()) {
+            return;
+        }
+
+        var context = Gdk.Display.get_default ().get_app_launch_context ();
+        var app = (Dock.App) apps.get_item (index);
+        app.launch (context);
     }
 
     private static void ungrab_keybindings () requires (instance != null) {

src/ItemManager.vala

@@ -6,11 +6,6 @@
  public class Dock.ItemManager : Gtk.Box {
     private static Settings settings;
 
-    private static GLib.Once<ItemManager> instance;
-    public static unowned ItemManager get_default () {
-        return instance.once (() => { return new ItemManager (); });
-    }
-
     public Launcher? added_launcher { get; set; default = null; }
 
     private DynamicWorkspaceIcon dynamic_workspace_item;
@@ -163,14 +158,4 @@
             warning ("Tried to move neither launcher nor icon group");
         }
     }
-
-    public void launch (uint index) {
-        if (index < 1 || index > AppSystem.get_default ().apps.get_n_items ()) {
-            return;
-        }
-
-        var context = Gdk.Display.get_default ().get_app_launch_context ();
-        var app = (App) AppSystem.get_default ().apps.get_item (index - 1);
-        app.launch (context);
-    }
 }

src/MainWindow.vala

@@ -23,6 +23,7 @@ public class Dock.MainWindow : Gtk.ApplicationWindow {
     private Pantheon.Desktop.Shell? desktop_shell;
     private Pantheon.Desktop.Panel? panel;
 
+    private ItemManager item_manager;
     private WindowDragManager window_drag_manager;
     private bool initialized_blur = false;
     private int border_radius = 0;
@@ -40,27 +41,27 @@ public class Dock.MainWindow : Gtk.ApplicationWindow {
         dock_box.append (new Container ());
         dock_box.append (new BottomMargin ());
 
-        unowned var launcher_manager = ItemManager.get_default ();
+        item_manager = new ItemManager ();
 
         // Don't clip launchers to dock background https://github.com/elementary/dock/issues/275
         var overlay = new Gtk.Overlay () {
             child = dock_box
         };
-        overlay.add_overlay (launcher_manager);
+        overlay.add_overlay (item_manager);
 
         var size_group = new Gtk.SizeGroup (Gtk.SizeGroupMode.BOTH);
         size_group.add_widget (dock_box);
-        size_group.add_widget (launcher_manager);
+        size_group.add_widget (item_manager);
 
         child = overlay;
 
         remove_css_class ("background");
 
         // Fixes DnD reordering of launchers failing on a very small line between two launchers
         var drop_target_launcher = new Gtk.DropTarget (typeof (Launcher), MOVE);
-        launcher_manager.add_controller (drop_target_launcher);
+        item_manager.add_controller (drop_target_launcher);
 
-        launcher_manager.realize.connect (init_panel);
+        item_manager.realize.connect (init_panel);
 
         settings.changed["autohide-mode"].connect (() => {
             if (panel != null) {
@@ -112,7 +113,7 @@ public class Dock.MainWindow : Gtk.ApplicationWindow {
         surface.compute_size.connect ((surface, size) => {
             // manually set shadow width since the additional margin we add to avoid icons clipping when
             // bouncing isn't added by default and instead counts to the frame
-            var item_manager_width = ItemManager.get_default ().get_width ();
+            var item_manager_width = item_manager.get_width ();
             var shadow_size = (surface.width - item_manager_width) / 2;
             var top_margin = TOP_MARGIN + shadow_size - 1;
             size.set_shadow_width (shadow_size, shadow_size, top_margin, shadow_size);
@@ -121,7 +122,7 @@ public class Dock.MainWindow : Gtk.ApplicationWindow {
         surface.layout.connect ((surface, width, height) => {
             // manually set input region since container's shadow are is the content of the window
             // and it still gets window events
-            var item_manager_width = ItemManager.get_default ().get_width ();
+            var item_manager_width = item_manager.get_width ();
             var shadow_size = (width - item_manager_width) / 2;
             var top_margin = TOP_MARGIN + shadow_size;
             surface.set_input_region (new Cairo.Region.rectangle ({