Enhance right-click options

[chrisgoringe]

Right0-click options should be:

When connected to a 'normal' chooser:

• progress this image (or progress as restart)
• cancel (when paused)

When connected to a multi-chooser:

• add this image (or remove this image)
• go (or restart from here)
• cancel (when paused)

When connected to the Fabric double multi-chooser:

• add this image to Positive (or remove this image to negative)
• add this image to Negative (or remove this image from negative)
• go (or restart from here)
• cancel (when paused)

See the UI mockup in https://github.com/chrisgoringe/cg-image-picker/issues/10

[chrisgoringe]

Things to consider:

• clicking one of the images brings it up alone - need to stop this for right click
• need to have the two nodes tightly linked, so the icons etc are always consistent

[chrisgoringe]

Done in new nodes