Misc: Miscellaneous changes

- Build separate object files in Makefile.

- Add preprocessor flag DEBUG_EVENTS to display info about received
  events.

- Fix the incorrectly inverted meaning of the "xinerama/showAll" option.

- Add a possible workaround for a weird issue I encountered with
  Xinerama that skippy-xd doesn't receive KeyRelease events on main
  window.

- Remove two unnecessary header file inclusions to drop dependency on
  libXmu.

- Rewrite wm_get_window_title(). Originally I wish to entirely rely on
  Xutf8TextPropertyToTextList(), but I found it even less tolerant about
  possible charset problems than the old approach, so we are still
  basically using the old method now.