Fix GTK keyboard-shortcut focusing of CTRL_LISTBOX.

Fix GTK keyboard-shortcut focusing of CTRL_LISTBOX.

I had put an entire piece of code into win_key_press's SHORTCUT_UCTRL
handler to carefully handle all the different kinds of list box
control and do something sensible with each one, and then I just went
and used a generic SHORTCUT_FOCUS type shortcut instead of actually
_calling_ all that carefully prepared code.

Now selecting (say) the character-classes list box in the Selection
panel using its Alt-e shortcut works; also, shortcut-selecting a popup
menu such as the ones in the Bugs panel causes the menu to pop up,
which I think is nicer than what previously happened.