ges: avoid use of G_DECLARE_FINAL_TYPE which requires GLib 2.44

Fixes gstreamer/gst-editing-services#48
6 jobs for dont-use-glib-2-44-api in 27 minutes and 13 seconds
Status Job ID Name Coverage
  Preparation
passed #70698
gst indent

00:00:18

passed #70697
manifest

00:00:35

 
  Build
passed #70699
build fedora x86_64

00:04:42

 
  Test
passed #70700
check fedora

00:14:24

 
  Full Builds
passed #70701
build android arm64 api28

00:03:51

passed #70702
build cerbero fedora x86_64

00:07:30