[clipper] Allow splicing of external video clips

[lae]

via lyger

another neat feature would be to allow the user to select an external video segment to use as an outro
or more robustly to allow you to define a segment
where you put something like external in the effect field
and the full filepath of the video file in the line body
and that segment could be at any position
the hard part is validating that the file is compatible (framerate, resolution)
and possibly converting it first if it isn't

[lyger]

For user friendliness, using "import" as the keyword may be more familiar.

Matching resolution/framerate could be done using ffprobe? Would those be the only compatibility issues?