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.)
Allocation rules:
* Use `as-gtk-allocation' when creating a Gtk widget that is the main
container for a given window<%> object. When the resulting
reference becomes unreachable, the widget will be released with
gtk_widget_destroy() through a finalizer.
* Use `atomically' to create and attach a sub-widget within the main
widget. Don't use gtk_widget_destroy(); the containing widget will
destroy the enclosing widget.
* For temporary objects, use `atomically' to wrap both the allocation
and release.
Every call to a function whose name contains "new" needs to be in one
of those cases.
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