Account for GtkApplicationWindow menu bar in geometry hints.

Account for GtkApplicationWindow menu bar in geometry hints.

This is another widget that can appear in the top-level window, in
addition to the drawing area and scrollbar we put there ourselves, and
hence which needs to be accounted for when figuring out the
relationship between the drawing area size in character cells and the
full window size in pixels.

Finding the menu bar widget itself is a bit of a hassle, but having
found it, dealing with it is basically the same as dealing with the
scrollbar, only with x and y swapped.