FindAdv: Try to add the possibility to search for accented characters in regex

FindAdv: Try to add the possibility to search for accented characters in regex

The problem is the handling of regex as using math-mode. That is
any accented character is converted to a math macro.
For instance "ä" --> "\\ddot{a}".
Outside of math or regex it is not converted (if used xetex flavour),
but there are other chars which are converted in math and in text (but differently)
For instance "ů"
	in math --> "\\mathring{u}"
	in text --> "\\r{u}"

TODO: determine the still not handled conversions.
It would be nice, if we could persuade math factory to not convert
these characters, but I was unable to find the place where the
conversion actually takes place.