Fix some annotation and cleanup unify `GESSource::create_source` vmethod

commit 694e732feabb11124daca308a09f30d87662e3d1 (HEAD -> annotation_fix, thiblahute/annotation_fix)
Author: Thibault Saunier <tsaunier@igalia.com>
Date:   Wed Feb 24 23:49:06 2021 -0300

    track-element: Fix and cleanup annotations
    
    Making the class subclass able by bindings

commit ddae1f6acfbe07d3af29bea184b394153a86a7b2
Author: Thibault Saunier <tsaunier@igalia.com>
Date:   Wed Feb 24 23:37:28 2021 -0300

    ges: Move GESVideo/AudioSource::create_source to GESSource
    
    Deprecating the old variants which were not introspectable
    and cleaning a bit the API.