Fix submenu size

!514 (merged) tried to fix long combo box mapping by avoiding the clamp-to-work-area logic that lives in gtkwindow.c. It didn't cover all the cases however, as gtkwindow.c in some situations tried again a few times to resize, except instead of clamping to the work area, it forgot about the size calculated by move-to-rect.

Fix this by "fixating" the size to the one received in the moved-to-rect signal until the menu is unmapped. This makes sure that a menu is not resized by the gtkwindow.c logic after it was configured by move-to-rect.