Add support for multiple displays per window

[rfuest]

This PR adds a new window type that supports to show multiple SimulatorDisplays in a single window.

Remaining issues:

• Input handling
• Displays that overlap the window's edge wrap around horizontally
• Make OutputSettings::framebuffer_size public to make it easier to layout the displays in a window
• Move max_fps from OutputSettings into Window and MultiWindow
• Better docs + example

[ost-ing]

Awesome @rfuest thanks for this work!

[rfuest]

I've updated the PR to support input handling. There are still a few remaining issues (see top), but the API should be fairly close to final and ready to test.