Write system tests

[Seeker04]

On top of unit tests (#21), we should also have high-level system tests.

I see three (not necessarily disjunct) approaches:

• xdotool(1) can send fake keyboard and mouse events from a script and can also check existence, geometry, flags, etc. of windows.
• Simulate Xorg by creating a mock version of plx.c.
• A System Testbook can be written for manual testing regressions after major changes.