Preview for individual clips

[owickstrom]

Right now you can only preview sequences and parallels, not individual clips. It would be very useful to preview individual clips, preferably implemented using the same machinery as the current preview.

Related to #11.

[Cmdv]

I took a look at this but couldn't quite work out some parts.

can see I need to add something like SomeVideoPart v -> ?? here:

komposition/src/Komposition/Application/TimelineMode.hs

Lines 335 to 341 in f4db3f0

	flatComposition :: Maybe Render.Composition
	flatComposition =
	atFocus (model ^. currentFocus) (currentProject model ^. timeline)
	Prelude.>>= \case
	SomeSequence s -> Render.flattenSequence s
	SomeParallel p -> Render.flattenParallel p
	_ -> Nothing

I need Maybe Render.Composition but I have VideoPart () so not too sure how to get to that.

Feel like something in this function should help here?