egl: Fix the installed headers

egl: Fix the installed headers

 • Use the public COGL_HAS_GLES[12] define instead of the HAVE_COGL_*
   ones which are private and defined in config.h,
 • Install clutter-egl-headers.h which is needed by clutter-egl.h,
 • Remove clutter-stage.h as it's uneeded and does not work since the
   single clutter.h include policy,
 • Install the egl headers into their own egl directory as the x11 and
   glx backends do. The include should then be <clutter/egl/clutter-egl.h>,
   so document it. It does not really break anything as nobody could
   have used those broken headers.