Fullscreen support

[longmathemagician]

This adds limited support for a borderless-window `fullscreen' mode on the current monitor.

Task & note list:

• Windows implementation: manual state caching, derived from Chrome's approach.
• MacOS implementation: implementation TBD, calling NSApplication::setPresentationOptions::NSApplicationPresentationFullScreen is likely sufficient although multi-window application support complicates this a bit.
• Linux - x11 implementation: relies on the window manager to cache state, works great under kwin but is untested elsewhere. WindowState getter and setter methods should probably be implemented before this is merged.
• Linux - GTK implementation: backend is currently broken. Is this a blocker?
• Linux - Wayland implementation: backend is currently broken.
• Web implementation: not sure if this is currently possible.
• Add builder support: not necessarily a good idea as fullscreen requests may fail, but should be here for consistency.
• Clean up example: derived from shello.rs, it's still longer than necessary.

[nyabinary]

GTK is now dropped