[Simple Timeline] Make video-only clips work
@bilboed
Submitted by Edward Hervey Assigned to Edward Hervey @bilboed
Description
Video-only clips are allowed to be dropped onto the simple timeline, but they cause mayhem in the composition since they don't have video.
When added through the simple timeline, we should link to them an blank-audio brother so processing happens correctly.
Imported from https://bugzilla.gnome.org/show_bug.cgi?id=441784