Reference the widget before returning it from ::create methods

Widgets (unlike Windows and Dialogs) returned by Gtk::Builder::get_widget*
start owned by the GtkBuilder object, the idea being that they will get
added to a container before the scope of the GtkBuilder object ends, and it
thus automatically gets destroyed.

But in the various ::create methods in pavucontrol, a pointer to the widget
gets returned, so that it can be added to a cointainer by the caller.
However as soon as the ::create method exits the GtkBuilder object owning
the widget, and thus also the widget gets destroyed, and we end up returning
free-ed memory.

This commit fixes this by making all ::create methods take a reference on
the widget before returning it, and having all the callers unreference the
widget after adding it to a container.

https://bugs.freedesktop.org/show_bug.cgi?id=83144
https://bugzilla.redhat.com/show_bug.cgi?id=1133339



Signed-off-by: Hans de Goede <hdegoede@redhat.com>