So that we can disable the combobox if the checkbutton is disabled,
rather than having inverted meaning, which can be very confusing.
Also make sure that the combobox has the correct state if we
have an unconfigured tablet.
When making the combo box unsensitive, disable the label too.
Also make sure that both widgets are correctly made
sensitive again when we have monitors in the drop-down.
Provides a barebones implementation of a panel for selecting the
display to map a tablet to. This implementation uses a combo box
instead of the Display-panel-like UI specified by the mockup since
my Cairo and GTK-fu are weak.
https://bugzilla.gnome.org/show_bug.cgi?id=668546
