panel: Allow restoring maximized/tiled windows by touch

Maximized and tiled windows can be restored with a drag gesture, not only from their titlebars, but also from any non-reactive parts of the top bar above the window. Currently this only works for actual pointer devices, extend the behavior to handle touch as well.

This is essentially in response to https://bugzilla.gnome.org/show_bug.cgi?id=770185#c18; I don't have any touch hardware around, so this is untested (apart from making sure to not break touchpad/mouse).