FindAdv: Polishing search with regex containing '.'

FindAdv: Polishing search with regex containing '.'

Also added missing math env alignat
Modified handling of longtable/tabular
Added a routine to count for valid chars. This is needed
for detection of word boundaries.

Due to detection conflicts
	regex '.*' vs match of word-boundaries in MatchStringAdv::operator()
we need to use '\b' in regex explicitly. E.g. '\b.*\b'

The backward search works, but
1.) only in current paragraph (this is the same as before)
2.) only in the same language environment.