Improve color map handling

[ghorwin]

Color plot/spectrogram plot property widget should offer a button to
select known color maps.

• known color maps are:
  • provided with software
  • read from 'colormaps' directory
• color maps are defined in relative value range (0..1)
• color maps are scaled to current value range in PostProc when applied

• user interface offers 4 buttons:
  • flip color map
  • select color map from list
  • select last used color maps (2 buttons)