Improvement: --blur-background-exclude #98 & MESA_swap_control & others

- Add --blur-background-exclude. (#98)

- Add `opengl-mswc` VSync, which uses MESA_swap_control instead of
  SGI_swap_control. I don't expect it to perform better than
  SGI_swap_control, though.

- Update CMakeLists.txt .

- Add a few targets for D-Bus `win_get`. Misc changes.

- Known issue: Apparently I've forgotten implementing --inactive-dim on
  GLX backend... Silly me.