Introduce MetaSettings and add the settings managed by MetaBackend into
the new object. These settings include: experimental-features and UI
scaling factor.
https://bugzilla.gnome.org/show_bug.cgi?id=777732
The UI scaling depends on whether the framebuffers are scaled. Enable
the caller to determine the what scale its UI should be drawn in, in
relation to the stage coordinate space by calling this function. A new
singal "ui-scaling-factor-changed" is added in order to liston for for
changes.
https://bugzilla.gnome.org/show_bug.cgi?id=777732
Implement MetaDnd for emitting DnD signals to plugins such as gnome-shell. The
xdnd handling code comes from gnome-shell, and it is hidden behind MetaDnd now.
https://bugzilla.gnome.org/show_bug.cgi?id=765003
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.
