Now Adv. F&R correctly interprets escape sequences within regexp insets, e.g., \...

Now Adv. F&R correctly interprets escape sequences within regexp insets, e.g., \\ or \(, for finding characters with a special meaning in regular expression context (\^()[]).
Added corresponding regression tests findadv-re-01 and -02 for finding these special characters.
Unfortunately, braces {} are a little bit special and don't work yet. Added findadv-re-03 regression test to not forget about this case.
This goes along the line of fixing issues reported by Andrew Parsloe and summarised as #7638.
Last note: Still there will be cases when the Advanced F&R won't work, especially if the text to be found and/or the search pattern is preceded by backslashes or others.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@39135 a592a061-630c-0410-9148-cb99ea01b6c8