Add PCRE regexps and convert file filtering to PCRE

I finally got PCRE 6.7 to build as DLL and integrated
with WinMerge.

This patch contains /Src changes to convert file
filtering code to use PCRE. Biggest changes is that we
must convert unicode strings to UTF-8 as PCRE supports
UTF-8. For that I created new files (Ucs2Utf8.h and
Ucs2Utf8.h) which contain conversion routines between
UCS-2 and UTF-8 (both directions). I didn't write those
functions, but copied them from some other file.

About PCRE. Luckily, PCRE builds without changes to
distributed sources. There is one (?) added header
file. And project files needed.

As usual with recent lib additions, I'll update build
instructions once I commit these changes.