add support for regular expressions (basic and extended)

it intruduces regex support in search engine
and filtering/searching in all screens

src/media_library.h

@@ -53,7 +53,7 @@ class MediaLibrary : public Screen<Window>
 		virtual void ReverseSelection() { Songs->ReverseSelection(); }
 		virtual void GetSelectedSongs(MPD::SongList &);
 		
-		virtual void ApplyFilter(const std::string &s) { GetList()->ApplyFilter(s); }
+		virtual void ApplyFilter(const std::string &);
 		
 		virtual List *GetList();