KWin support

[flyingpie]

Initial support for KWin, the compositor for KDE Plasma.

This is mostly a proof-of-concept, to get a better idea of what refactors are necessary in the rest of the app.

I'm merging it now, even though it's still a rough implementation, because:

• It's annoying to drag around a large-ish branch, constantly updating stuff happening on master;
• It's a mostly isolated set of code, not polluting the rest of the app;