Now that the keyboard panel knows how to handle reverse shortcuts, we
can start annotating the existing ones with the needed XML attributes.
This commit does that with switch-input-source{-backward}.
Note that some changes in some modules are needed for this to work
correctly.
In this case, a default value needs to be set for
switch-input-source-backward in gsettings-desktop-schemas, and the
Meta.KeyBindingFlags.REVERSED flag needs to be removed from the
shortcuts defined in gnome-shell source. Instead of having
mutter handle the reversion with 'shift' by itself, it's now handled
explicitly through gsettings keys.
https://bugzilla.gnome.org/show_bug.cgi?id=731618
