keyboard: Add the IBus engine setting code

We simply connect to IBus and tell it to switch engine when our
current input source setting is changed and an IBus engine is
specified there. The responsibility to make sure that this engine
actually exists on the IBus side is left to whoever writes the
setting.

At the same time, if an IBus engine is specified, we flip the setting
that backs the Gtk/IMModule XSETTING so that GTK+ applications get
notified to load the IBus input method when needed and go back to the
simple input method when IBus isn't required.