Improvement: Change VSync mode with D-Bus & Makefile update & Misc

- Add on-the-fly VSync option modification via D-Bus, as requested by
  kunitoki (#80). Expose parse_vsync(), create vsync_init() and
  ensure_glx_context().

- Change default value of ps->drm_fd to -1.

- Update Makefile. Change the install/uninstall rules and add doc
  installation, requested by hasufell in #85.

- Mark window not damaged in map_win(). It helps in reducing flickering
  with inverted window color, but I'm not completely sure if it's safe.

- Avoid modifying w->invert_color when window is unmapped.

- Update documentation. Thanks to hasufell for pointing out.