Preview panel like insight

This is NOT READY to be considered for merging yet. I'm opening the PR so that everyone can follow progress. Summary of work:

 - Web preview panel has same (or very similar) rendering controls as Insight
 - All changes are 'applied' immediately (no "apply" button).
 - Reset, min/max and full range settings are not saved and can be reverted by undo/redo.
 - Only on 'Save' are the changes saved and thumbnail updated.
 - The main image viewer rendering panel is also more compact (looks neater and allows more code reuse with Preview panel).