Refactoring GESVideoSource so that #GESUriVideoSource can handle
still image in a simple way
MultiFileSource has been replaced with the new `imagesequencesrc`
element, this was totally broken anyway as `multifilesrc` can not seek
properly.
This way the user can easily know how the clip would look like
if no scaling was applied to the clip, this is useful to be able
to properly position the clips with the framepositionner element.
Basically we know that if we are using mixing, compositor will be
able to do video conversion and scaling for us, so avoid adding those
usless elements.
This optimizes a lot caps negotiation for deeply nested timelines.
We need different export decorators for the different libs.
For now no actual change though, just rename before the release,
and add prelude headers to define the new decorator to GST_EXPORT.
With two exceptions:
* ges_clip_create_track_elements_func
* ges_uri_clip_set_uri
which were never declared in headers and should always have been static.
