popupMenu: Adjust submenu arrows to RTL icon changes

Since commit e04e507659027, we will already get the right icon for the
submenu arrow, so we must not mirror it again. However we do need to
take the text direction into account for the rotation now (but that's
not actually too bad - the resulting code gets quite a bit easier).

https://bugzilla.gnome.org/show_bug.cgi?id=736343