[Future] UI controls

[pcuenca]

After our conversation last night I was thinking about the UI and came up with the following, for further discussion:

• Remove the Point / Foreground dropdowns.
• Right click -> Add point (foreground)
• Left click -> Remove area (background point). Ignored if there are no points.
• Drag gesture -> Box (always foreground).
• Ctrl-click -> Create a new mask, add that point, and auto-assign a new color to the mask.