This refactoring of the color/opacity preset management does the
following major improvements/changes.

1. Preset management is moved to ServerManager so that it is available
   from Python clients.
2. Preset management now supports opacity transfer functions.
3. New preset loading dialog adds ability to search for presets using
   their name and several other usability improvements.
4. Using a JSON based format for presets which is consistent with the
   format used in "settings" for proxy defaults. This avoid having to
   describe a different XML format for color maps. Loading old style
   color maps is still supported.
5. A tool to convert legacy color map XML formats to new style JSON.