Selections Masks

[stdgregwar]

This mask selection features should match theses points

• having a current collection of selection masks from 1 to (1)0
• user should see current selection mask in the UI
• shortcuts for switching selection easily
• being able to use selection as input for transforms or other plugins
• use different color for each selection masks
• allow to display multiple selections masks at once
• use positional convention to forward masks to algorithms