keyboard: Add support for hidden keybinding XML data

If a KeyListEntry has a hidden="true" attribute, then the corresponding
binding information will be loaded as usual, but the binding won't be
displayed in the user interface.

This is useful as the keyboard panel will take into account hidden
keybindings when detecting conflicting shortcuts, or to suggest to set a
reverse shortcut.

For now, this will be used for the various reverse mutter keybindings
({switch,cycle}.*-backward) as they should not be shown in the UI, but
we still want the keyboard panel to know about them.

https://bugzilla.gnome.org/show_bug.cgi?id=731618