To enable mouse events to a panel, go back to using a
GtkEventBox for a panel, but make it non-visible so that
it doesn't interfere with the parent's background color.
The extra layers added in the previous commit are still
important to avoid turning that into a native window
(which can completely breaks background drawing). Finally,
add an extra layer just around tab panels so that events
get delivered to the tabs. (It's possible that the extra
layer will make the background wrong just around the tabs,
if the tabs don't fill the area above the panel; too bad.)
Restore the use of freeze & thaw on transparent canvases, but
ensure that the implementation underlying the GDK window won't
change while a freeze is in effect.
The main advantage of GTK+ 3 is better support for HiDPI
displays. If GTK+ 3 libraries are not available or if the
`PLT_GTK2` environment variable is defined, GTK+ 2 is used
as before.
