Image viewer split 11285

This allows more than 1 channel to be active when showing split-view layout in the web image viewer (similar behaviour as Insight).

To test:

 - Open multi-channel image in web image viewer - show Color mode.
 - Go to 'Split Channel' viewing option.
 - Change to greyscale - Current channels stay on (in grey) & merged panel is shown.
 - Change to 'Normal' viewing option.
 - Only a single greyscale channel should be shown - buttons will toggle channel as normal.

 - Try to play with different combinations of channels & viewing option to check that switching between them gives predictable behaviour. 
![screen shot 2013-09-09 at 23 20 54](https://f.cloud.github.com/assets/900055/1110982/63077fc4-199e-11e3-9b58-217142e5c1bd.png)