core/window: add startSystemMove, startSystemResize, maximize, and fullscreen

[bbedward]

Allows to trigger move and resize operations on QsWindow, useful for making a built-in CSD, etc.

Also expose maximize and fullscreen methods and bindables.

[outfoxxed]

Why is this exposed on ProxyWindowAttached and not FloatingWindow? Its illogical for other window types

[bbedward]

Why is this exposed on ProxyWindowAttached and not FloatingWindow? Its illogical for other window types

Why is this exposed on ProxyWindowAttached and not FloatingWindow? Its illogical for other window types

I shifted it all to FloatingWindowInterface