keyboard: Add 'reverse' metadata to switch-input-source shortcuts

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

panels/keyboard/01-input-sources.xml.in

@@ -4,9 +4,12 @@
                 _name="Typing">
 
   <KeyListEntry name="switch-input-source"
+		reverse-entry="switch-input-source-backward"
                 _description="Switch to next input source"/>
 
   <KeyListEntry name="switch-input-source-backward"
+		reverse-entry="switch-input-source"
+		is-reversed="true"
                 _description="Switch to previous input source"/>
 
 </KeyListEntries>