In a gdm session, we may not know what mouse orientation the user
may be in, so it makes sense to support both the left and right
mouse buttons to activate login or other items.
Additionally, add the behavior to all modal dialog items, even in
a user session, because it's unlikely that the user will right-click
on buttons, and it makes for an easier implementation.
https://bugzilla.gnome.org/show_bug.cgi?id=688748