clutter currently never emits activated or deactivated signals on
the stage object when using the EGL backend. Since the stage never
gets activated, accessibility tools, like orca, don't work.
This commit makes mutter take on the responsibility, by tracking
when the stage gains/loses focus, and then synthesizing stage
CLUTTER_STAGE_STATE_ACTIVATED state events.
A limitation of this approach is that clutter's own notion of
the stage activeness won't reflect mutter's notion of the
stage activeness. This isn't a problem, in practice, and can
be addressed in the medium-term after making changes to
clutter.
https://bugzilla.gnome.org/show_bug.cgi?id=746670
This allows creating the stage much earlier than it otherwise would have
been. Our initialization sequence has always been a bit haphazard, with
first the MetaBackend created, then the MetaDisplay, and inside of that,
the MetaScreen and MetaCompositor.
Refactor this out so that the MetaBackend creates the Clutter
stage. Besides the clarity of early initialization, we now have much
easier access to the stage, allowing us to use it for things such as
key focus and beyond.
