Improvement: ARGB window matching & Enable track_focus with D-Bus

- Add predefined matching target "argb" to match ARGB windows.

- Make it possible to enable focus tracking on-the-fly with D-Bus
  method.
Bug fix: Create OpenGL context only after initializing overlay

- Create OpenGL context only after initializing overlay, which fixes a
  bug that --paint-on-overlay does not work with OpenGL VSync. Thanks to
  tsmithe for reporting. (#7)