Use new meta_key_binding_is_reversed() method

Now that mutter gives a way to check if a MetaKeyBinding was marked as
'reversed' or not, gnome-shell does not have to hardcode that a
MetaKeyBinding using a shift modifier is reversed, it can directly check
if the appropriate flag is set.

https://bugzilla.gnome.org/show_bug.cgi?id=732296
Stop using Meta.KeyBindingFlags.REVERSES for IM switch keybinding

When this flag is set on a MetaKeyBinding, mutter will know that
the keybinding has an associated reverse keybinding triggered with
the shift modifier. However, an undesirable side-effect is that
gnome-control-center keyboard panel does not know that this 'shift'
is reserved for these reverse keybindings and cannot detect
conflicting bindings in this case.
This 'reverse' logic can now be handled at a higher level (in gcc keyboard
panel) so this commit removes it from gnome-shell so that they do not
conflict.

https://bugzilla.gnome.org/show_bug.cgi?id=732296
Remove 'backwards' argument from SwitcherPopup:_keyPressHandler

All derived classes are already checking explicitly for action names
(FOO and FOO_BACKWARDS). mutter used to have a META_KEY_BINDING_REVERSES
flag for keybindings which required special handling of "shift"+FOO as
FOO_BACKWARDS, but this has been removed now, so this special handling
is no longer necessary.

https://bugzilla.gnome.org/show_bug.cgi?id=732296