Add ripple option to speed effect

When reducing the speed of a clip its length grows and sometimes there is a another clip in the track that might prevent from using the desired effect. It would be handy to add a feature to ripple clips in the track after the applied spped to a clip. Sorry if this seems confusing, couldn't really find a way to explain it.

• add ripple edit -> see #224 (closed)
• add set video speed to take a specific time

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set video speed to take a specific time: https://bugs.kde.org/show_bug.cgi?id=419612