Separated task planning and processing into separate transformations
* "plan.xsl" traverses the file-tree provided by "list.xsl" and determines the tasks to be executed
* "process.xsl" executes the tasks planned by "plan.xsl" in a sensible order
* this change was implemented to be able to e.g. schedule the linkage tasks for last
** performing them in tree-order caused problems when the generator tried to create symlinks inside non-existing directories
** additionally this further modularizes the processing chain