Support merged channels in viewer

[urubens]

Add an option to merge channels in the viewer, or to see channel per channel. The current behavior is inconsistent as:

• original RGB images are only visible with merged channels
• other images are only visible channel per channel

It has to be possible to go from one mode to another for all images.

It raises the question on how to handle annotations in these different modes and existing annotation backend limitation. Proposal as temporary solution:

• RGB image:
  • Merged:
    • Annotation read: OK (existing)
    • Annotation write: OK (existing)
  • Split:
    • Annotation read: OK (as 1 annotation is on all channels)
    • Annotation write: OK ? - If ok, display a warning box ?
• others images:
  • Merged:
    • Annotation read: OK (show all annotations from the Z,T slices)
    • Annotation write: Disabled ?
  • Split:
    • Annotation read: OK (existing)
    • Annotation write: OK (existing)

[urubens]

Implemented in 86e76e72cb7618433b3e12db33ac02d713e62174

Regarding the issue, these choices have been made:

• RGB image > split > annotation write: OK - no warning
• other images > merged > annotation write: Disabled draw tools + message tooltip on draw tool hovering.