FindAdv: If possible use unicode instead of math command when searching

FindAdv: If possible use unicode instead of math command when searching

Wrong behaviour before this commit:
Open new lyx-file
insert
	unicode-insert 0x025c 0x1d08
Open adv-find dialog
search for unicode 0x025c OK
search for unicode 0x1d08 OK
search with regex for unicode 0x025c OK
search with regex for unicode 0x1d08 ==> NOT OK (because we are searching for \textrevepsilon
	and this was mapped to 0x025c