Rework window / actor focus handling

The duality of the Clutter's key focus and mutter's window focus has long been
a problem for us in lots of case, and caused us to create large and complicated
hacks to get around the issue, including GrabHelper's focus grab model.

Instead of doing this, tie basic focus management into the core of gnome-shell,
instead of requiring complex "application-level" management to get it done
right.

Do this by making sure that only one of an actor or window can be focused at
the same time, and apply the appropriate logic to drop one or the other,
reactively.

Modals are considered a special case, as we grab all keyboard events, but at
the X level, the client window still has focus. Make sure to not do any input
synchronization when we have a modal.

At the same time, remove the FOCUSED input mode, as it's no longer necessary.

https://bugzilla.gnome.org/show_bug.cgi?id=700735