[pitivi] Support the notion of placeholder/empty clips and allow manually swapping clips
Submitted by Jeff Fortin Tam @nekohayo
Link to original bug (#676388)
Description
Let's say we have a project with clips A, B, C, that are all in the source list (pitivi's media library) and in the timeline.
For some reason, clip B is temporarily unavailable on the hard drive or the user has deleted it.
AFAICT, currently there is no mechanism to handle this; either the project loads fully with all related media, or it fails completely.
Ideally, we should allow loading a project and, if the user cannot immediately provide a replacement for the missing clip "B", tell the application that this clip B is to be considered a placeholder. Then, the application could show it as such in its media library and in the timeline.
Later on, the user could either delete the placeholder or provide a replacement for it (ex: right click, -> "Replace clip...").