enhance unique value dialog:

- allow selection of multiple classes
- new 'add class' button (fixes #1099)
- new 'reset colors' button to reset the color of selected/all classes to white
- new 'randomize colors' button to randomly set colors of selected/all classes
- 'remove classes' removes selected classes
- change common attributes for multiple selected classes at once
- allow renaming of classes
- 'Classify' now adds missing instead of recreating all classes
- used QMap instead for std::map for classes


git-svn-id: http://svn.osgeo.org/qgis/trunk@8493 c8812cc2-4d05-0410-92ff-de0c093fc19c