Separated preview drawing into own ARegion, this should make using View2D possible
The Sequencer now has three view types: Sequencer, Preview and split Sequencer/Preview.
Changing the preview can be done either by the combobox in the header or toggling through those types with CTRL+TAB.
Notes:
* Icon for split Sequencer/Preview view missing still.
* Naming items in the comboboxes can be improved (just Preview instead of Image Preview?)
Next steps:
* bringing back View2D handling (zoom/pan) for image preview
* experimenting with splitting the Preview ARegion for In/Out editing
18cc2b76c2