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.
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