Add meta_key_binding_is_reversed()

MetaKeyBinding can be marked as being reversed
(META_KEY_BINDING_IS_REVERSED), but MetaKeyHandlerFunc callbacks
cannot check whether this flag was set or not on the MetaKeyBinding
which triggered the callback.

https://bugzilla.gnome.org/show_bug.cgi?id=732295
Don't automatically add bindings for -backward actions

Currently the bindings for {switch,cycle}.* actions are created with the
META_KEY_BINDING_REVERSES flag so that <shift>+binding triggers the
reverse action. However, gnome-control-center does not know about this
kind of implicit bindings, and, for example, cannot warn when the user
tries to setup a conflicting <shift>+xxx binding.

These backward <shift> bindings are being explicitly set in
gsettings-desktop-schemas, so the META_KEY_BINDING_REVERSES annotation
can be removed for them from mutter.

https://bugzilla.gnome.org/show_bug.cgi?id=732385
Add hidden -backward bindings to 50-mutter-navigation.xml

This makes the gnome-control-center keyboard panel aware of these
bindings so that it can warn about conflicting bindings if the
user tries to use one of these bindings for a different action.

https://bugzilla.gnome.org/show_bug.cgi?id=732385
Remove use of META_KEY_BINDING_REVERSES

Now that the internal mutter bindings and gnome-shell stopped using
META_KEY_BINDING_REVERSES, and after moving the 'adding shift reverses
the keybinding action' logic to gnome-control-center, we can remove
META_KEY_BINDING_REVERSES from mutter.

Plugin API is broken as this constant is removed from the exported
headers. ABI is broken as using this flag is now a noop.

https://bugzilla.gnome.org/show_bug.cgi?id=732385