Allow users to move existing cutmarks

[oas777]

Together with #107, this is necessary for precision cuttting.

[lkiesow]

Can you elaborate a bit more on what you would like to see?

[oas777]

Once you have set cutmarks (or cutmarks were suggested based on audio analysis), this features allows you to move the cutmarks a few frames or seconds (especially when zoomed) to correct them. Which is not possible right now:

[lkiesow]

Additional/alternative description in #1228

[lkiesow]

Fixed by #1315