- refactored colour selectors to generic functions and controls
- added colour selectors for outlines
- removed alpha settings from colour selectors, since they are shared a
  separate shared control should be added


git-svn-id: svn://svn.code.sf.net/p/gwyddion/code/trunk/gwyddion@16701 25697fe4-d644-49b8-9d30-be5465f2adf3