[TODO] Implement new flag handling

[inxomnyaa]

Description:

Flags that can be used in commands, for example -hollow

Every action, shape and command may have different flags

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TODO list:

• Come up with a simple, yet useful way to register flags
• Enable the command handling & suggestions
• Global / static edit flags? i.e. "always hollow" when editing
• Make sure brushes + brush ui still work
• Wiki