NEW FEATURE: Implement better selection of colormap

Summary

There are way more colormaps available and it would be nice to select them independent from the current push button

Reason

more freedom

Detailed explanation

One possibility (and probably the easiest) would be to include a colormap dialog as it is already implemented for psy-simple (see Screenshot below) for the psyplot-gui. @tobstac: Do you think this is appropriate or do you have any other ideas?

Examples