This patch tries to make masking UI in Image Editor match masking in Clip Editor as close as possible.
- The Mask Display menu is now part of the header just like in MCE
- The mask options are no longer part of Image category but now have their own Mask tab
- When you are in Mask mode you can call the same masking context menu just like in MCE
What's missing are the tools themselves, which are in the old toolshelf are in MCE, but since we do have active tools in image editor, the only option would be to put them in the sidebar on the right. So I am leaving those out.
I know it's a bit late and UI is frozen already, but since those options in MCE just got into master yesterday I figured it should be fine :)