gstreamer-sharp: Fix App{Src,Sink} constructors

gstreamer-sharp: Fix App{Src,Sink} constructors

Apparently GtkSharp expects each object has only one ToggleRef at any
time. Assigning element.Handle into Raw has a consequence that second
ToggleRef attempts to get created but fails on g_object_unref () that
breaks a GObject assertion:

  toggle_refs_notify: assertion failed: (tstack.n_toggle_refs == 1)

This is because toggle references should be removed with
g_object_remove_toggle_ref(), not a simple unref().

In order to avoid duplicate toggle references, introduce
ElementFactory.MakeRaw(), which creates a GstElement without its
accompanying C# object. The returned raw pointer can be assigned into
another GLib.Object without trouble.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1885>