- implemented 'fall back' support so that if a theme is missing icons, the default theme icons will be used instead.
- use a static function in qgisapp to get theme icons and pixmaps
- updated all icon fetching code to use the new methods above
- updated qgsapplication so that it differentiates between active and default themes
- fixed a bug in qgsoptions that prevented themes being changed in some cases.
This is a part fix for #1054 - still need to do some work to make GRASS theming work properly
git-svn-id: http://svn.osgeo.org/qgis/trunk/qgis@8937 c8812cc2-4d05-0410-92ff-de0c093fc19c
