Improve colour map interaction

[NickDraper]

Ticket
http://trac.mantidproject.org/mantid/ticket/11629

To test

Note this should only affect 2D color map plots

1. start mantidplot
2. load up some data (& rebin if event)
3. Create a color map plot
4. Double click within the plot data
5. Select Layer Details
6. Pick the colors tab
7. Play with the image colour options
8. check you can set all of the options, and for custom color maps you can set them as the default.
9. check the default is applied to new colour maps and is persisted if mantidplot closes.

[FedeMPouzols]

It works well. The new functionality works as expected, and all the widgets in the 'colors' tab seem to behave as they should (tested on debian).