hid/gtk: Create a GtkToolbar of mode buttons for compact vertical mode
Rather than packing our mode buttons into an hbox, use a proper GtkToolbar
so theming will match the rest of the user's desktop.
This simplifies code to switch compact mode on and off, as we have two
distinct widgets to show / hide, rather than having to reparent the mode
buttons each time.
It does, however mean we have to be careful not to double-trigger events
when keeping the two sets of mode buttons in sync with each other, so that
isn't ideal.
In the longer term, we probably ought to split out the mode button area
on the left-hand toolbar as a separate self-contained widget.