Multiscreen configurations: specify which screen to use for the Graphite window

[BrunoLevy]

Add a configuration variable plus a GUI in the preferences.
Make sure the window opens on the main screen if specified screen is non-present (for laptop users when they work in the train !)
(From @gloook)

[BrunoLevy]

Argh, it seems that the monitor parameter for GLFW window creation is only for fullscreen windows, I don't see how to make a windowed window that appears on a given monitor...

[BrunoLevy]

OK, I understand:

• when specifying a monitor for window creation, window is systematically fullscreen
• one can move a window to a specific monitor using glfwSetWindowPos(). It is possible to find where a given monitor is mapped using glfwGetMonitorPos(). Claude helped me figuring out that

[BrunoLevy]

... does not seem to work under Windows (tested by @gloook)