Originally intended commit message from http://trac.wildfiregames.com/attachment/ticket/2951/template_subsets.diff

Make special templates (foo|templatename) (apart from actor|) more moddable. Fixes #2951.

This moves template additions to xml files.
Permitted components are now done via filtered and merge.
For adding new nodes while ignoring possible previous values use replace.

Some explanation on how to use the new filter templates:

• Previously permitted component types should be replaced by <Foo merge=""/>, which does not add the <Foo> node if it wasn't present.
• To actually remove the other components the encompassing node should specify filtered="", which will remove all nodes which are present in the original, but not in the filter.
• Additions that should be performed while ignoring a possible original node should specify replace="".
• To add something to an existing node (or add that node if it does not exist) just use <Foo>...</Foo>.