pitivi: 0.95 -> 0.96 (fixes startup error) (#17228)

* pitivi: 0.95 -> 0.96 (fixes startup error)

The upgrade of gtk3 to v3.20 broke pitivi, causing it to segfault after
the user closes the Welcome dialog. This release fixes that.

New dependencies: gst-transcoder and dbus.
(pitivi imports dbus if it finds the GNOME_DESKTOP_SESSION_ID
environment variable. Without dbus there will be some ugly errors for
GNOME desktop users.)

* pitivi: use wrapGAppsHook instead of makeWrapper

Allows us to remove preFixup.

* pitivi: move meta section to the end

Like most Nix package files are written.