Support fullscreen on X11

[jakeisnt]

• [ ] EventWindowFullScreenEnter
• [ ] EventWindowFullScreenExit
• [x] Window::setFullScreen(boolean)
• [x] Window::isFullScreen()

Unhandled:

• Whether fullscreen should lock exclusive fullscreen (i.e. hiding background pop-ups). I assume that exclusive fullscreen is desirable, but left a hardcoded flag and documentation.
• Whether window decoration flags should be handled separately from full-screen (X11 can add and remove window decorations, like borders and the header, without changing the window size - is it safe to assume window decoration state doesn't change?)
• On XWayland, there is a minor amount of screen tearing visible. I was unable to track this down (and it may not be a problem on X11), but it's perfectly functional.

[jakeisnt]

The two Events aren't explicitly supported by X11 - still tracking them down. OK to merge without them.

[jakeisnt]

Helps resolve #130

[tonsky]

Works for me on Ubuntu 20.04. Thanks!