Added FxData#setIndex to support efficient in-place sorting of groups and properties (without using positions) by external callers.

git-svn-id: http://svn.flexive.org/repository/flexive/trunk@2997 d35d4baa-7912-0410-b0ae-edacad209e15
FxGroupData#setIndex, applyIndices: minor performance improvements

git-svn-id: http://svn.flexive.org/repository/flexive/trunk@2998 d35d4baa-7912-0410-b0ae-edacad209e15