Implement a keep-up triangle to keep submenus from changing if the
pointer strays a bit when mousing towards the application list on the
right. The keep-up triangle is only used if the mouse movement is more
horizontal than vertical.
This is similar to the behaviour of GtkMenu, but much more simple
because we are only dealing one specific kind of menu, and hence don't
need to be as generic.
https://bugzilla.gnome.org/show_bug.cgi?id=692913
802c70e14c