Check gdk_event_get_scroll_deltas() when scrolling menus. This fixes
unusual scrolling behavior on Mac when using trackpad or magic mouse
smooth scrolling on large popup menus.

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=13827
Merge pull request #4 from bratsche/menu-scrolling

Check gdk_event_get_scroll_deltas() when scrolling menus.