This is a lump commit that is fairly difficult to break down without
either breaking bisecting or breaking the test cases.

The new design for handling X11 event translation works this way:

  - ClutterBackend::translate_event() has been added as the central
    point used by a ClutterBackend implementation to translate a
    native event into a ClutterEvent;

  - ClutterEventTranslator is a private interface that should be
    implemented by backend-specific objects, like stage
    implementations and ClutterDeviceManager sub-classes, and
    allows dealing with class-specific event translation;

  - ClutterStageX11 implements EventTranslator, and deals with the
    stage-relative X11 events coming from the X11 event source;

  - ClutterStageGLX overrides EventTranslator, in order to
    deal with the INTEL_GLX_swap_event extension, and it chains up
    to the X11 default implementation;

  - ClutterDeviceManagerX11 has been split into two separate classe...