Improvement: A number of changes

- Add Ctrl+(Shift+)Tab binding to cycle through windows. It doesn't
  cycle in the correct order, though. (#27)

- Add allowUpscale configuration option. (#24)

- Focus mini window of initially focused window on start. This actually
  breaks movePointerOnStart, but... Does anybody use it? (#26)

- Stop grabbing keyboard. After all, the focus issue couldn't be
  resolved too well even if we grab the keyboard.

- Add a bit comment to layout.c, which I don't understand yet, either.
  :-D

- Fix compiler warnings.