Node preview merge function now has an additional option "remove_old", which could be used if previous previews should be retained after recalculating (not used currently, old previews are dropped entirely for now, because it could be confusing).

Also moved the sync and merge for previews into the respective tree callbacks for consistency. This way the handling of previews can be changed for each tree type individually if necessary.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55502 954f8c5b-7b00-dc11-b283-0030488c597c