Timeline zooming/panning improvements

[CDx4f3kCAf3Y]

This is in commit d6ea102.

Currently, the Fit -/+ buttons can be used to zoom in/out on the timeline.

However, to pan to a later time (at the same zoom level), the timeline needs to be zoomed out, the playhead moved, and then the timeline zoomed back in.

Some ideas come to mind:

• Make the timeline scrollable horizontally
  • This would allow the timeline to be panned left/right using the horizontal scroll bar and/or shift+scrollwheel.
• Make the timezone zoomable using ctrl+scrollwheel.

[CDx4f3kCAf3Y]

This is working great in 1f6ac98. Thank you!