On OSX the framespace and CSD (Client-Side Decorations) are not
supported at all... I am not able to test this case. This patch
restores the main menu functionality based on pre 1.19 themes,
where it was located inside win.edc (app content) and not in
border.edc (framespace).
Note that the initial size of a window may be wrong, eg as in
elementary_test -to "Main Menu"
Fixes T5734 (hopefully!)