Waveform editor improvements

[cavis]

The waveform UI is now involved in stream-recording. Some improvements may be in order to make it easier working with 1 hour clips.

Laundry list of ideas (not saying we need to do all of these):

• Start playback position at end of preroll slice
• Spacebar to play/pause, if any part of the waveform UI is focused.
• I and O for marking in and out points
• Other keyboard shortcuts for marking breaks? (Vs pre/post slicing)
• Arrows to nav left-right.
• Maybe also mouse scroll left-right? And the usual DAW zoom controls? Overkill?
• Maybe a different UX to allow users to "select" the timeframe they want to crop to?
• Pre/Post are kind of misnomers for the recorded hours. More like, "this is where we think the top of the hour is".
• The timestamp 00:00 numbers aren't editable. Are they just a background or something?
• Allow zooming all the way out
• Update view while dragging the horizontal scrollbar
• An actual playhead instead of the change in color
• Some sort of bug I haven't tracked down yet ... clips copied to episodes always look like their segmentation "changed" to the media page. Naving away warns you about unsaved changes.