This adds support for "generator effect strips", which don't need necessarily
an input strip and my version of Matt Ebb's [ #5035 ] 'Solid Color'
sequence strip.
TODO: With a little bit more tweaking it will be possible to make animated
effect plugins and my still unfinished "Bake"-Strip.
For the 'Solid Color'-Effect, to quote Matt:
This is nice and simple, just provides a solid colour that's set in a colour picker in the properties popup. This is something we've needed for a long time, and I got totally sick of having to make 'black.png' and 'white.png' just to do fades, so I coded this.
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