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 classes,
    one that deals with core and (optionally) XI1 events, and the
    other that deals with XI2 events; the selection is done at run-time,
    since the core+XI1 and XI2 mechanisms are mutually exclusive.

All the other backends we officially support still use their own
custom event source and translation function, but the end goal is to
migrate them to the translate_event() virtual function, and have the
event source be a shared part of Clutter core.