Feature Request: apply slow/fast motion without prior rendering

[metux]

Support applying tools like slow/fast motion directly on a clip (and allow changing those settings afterwards),
without having to render/export the clips and add them as new media first.

Rationale: it's currently quite complicated and time consuming to do those things, especially when having to
do it with many pieces of a movie. Optimally, it should be possible to just apply those effects (similar to filters etc)
directly on clips and do the actual rendering only in the very final production state.