ges-video-transition: add a new "fade-in" transition type

ges-video-transition: add a new "fade-in" transition type

This transition is meant to be very similar to crossfade, but
instead of fading out the background video at the same time as the
foreground fades in, the background video stays at 100% opacity
during the whole transition.

This essentially "restores" the old crossfade behaviour that was changed in:
https://gitlab.freedesktop.org/gstreamer/gst-editing-services/-/commit/eb48faf34234811faec1699692a2c096daaceefd
but using a new type enum, so that both behaviours are available,
letting applications choose.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2385>