Fix menu placement to avoid dead xinerama areas, possibly break other stuff

Use screen_find_monitor(area) instead of screen_find_monitor_point(
topleft corner) in order to find a better monitor when the menu isn't
opening with the mouse cursor in the top left corner.

I made screen_find_monitor return the primary screen when it failed to
find a monitor containing the rect, instead of the total area, no idea
what behaviour this will change but I doubt it will be worse.