popupMenu: changes rotation center of sub-menu's triangle

When the triangle rotates (when sub-menu is expanded), it seems as if
the triangle pivots from one corner even though rotation center is set
to Clutter.Gravity.CENTER. Hence the rotation center is set nearer to
the edge than to the corner ([0.3, 0.5] instead of [0.5, 0.5]) so that
it doesn't appear odd.

Also pivot_point is used instead of rotation_center_z_gravity as it is
deprecated.

http://bugzilla.gnome.org/show_bug.cgi?id=703109