The placement of this function in tm_source_file is not right - by moving
it to the workspace we can make the source file unaware of the existence
of the workspace (no inclusion of tm_workspace.h in tm_source_file any
more). Also change tm_source_file_new() so it doesn't offer the source file
update.

After this change
* TMWorkspace knows TMSourceFile and TMTag
* TMSourceFile knows TMTag
* TMTag knows TMSourceFile