On X11, the position of the menu is calculated synchronously by
gdk_window_move_to_rect(). This means that calculating the window size
when showing is too late, as that'd mean the size used when calculating
the position is out-of-date. The first time a menu is mapped, however,
the size is calculated during realization; but a window is only realized
once, so it doesn't work for subsequent maps.

Currently, this is harmless, as a GtkMenu can change its size however it
wants after it has been mapped. This, however, is problematic, as it
means the position calculated by gdk_window_move_to_rect() might no
longer be valid, or constraints made by the same function might no
longer be respected.

Thus, this is a preparation for making GtkMenu popups stay the same size
until they are remapped again at a later point.