popupMenu: Force submenu arrow rotation angles to multiples of 90

When you open or close a PopupSubMenu the arrow icon rotates, but
the code previously assumed that the starting angle was a multiple
of 90.  If you click the submenu fast enough the rotation could overlap
with the previous rotation and you'd get something that wasn't a multiple
of 90.
Now we ensure that the ending arrowRotation angle is always a multiple
of 90 regardless of what the starting angle is.

https://bugzilla.gnome.org/show_bug.cgi?id=728927