Another try at fixing bug 93 "tilde signatures inside nowiki tags sometimes get expan...

Another try at fixing bug 93 "tilde signatures inside nowiki tags sometimes get expanded (<includeonly><nowiki>~~~~</nowiki></includeonly>)"

* Change the preprocessor to insert strip items for <ignore> nodes during pre-save-transform, just like <comment> nodes are handled already. This effectively disables all pre-save-transform steps inside <includeonly> tags.
* Adapt parser tests to the new behavior.