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: gst-editing-services@eb48faf3 but using a new type enum, so that both behaviours are available, letting applications choose.