song/StringFilter: add flag substring

Prepare to stop using substrings for filter expressions.

src/song/StringFilter.cxx

@@ -32,8 +32,12 @@ StringFilter::Match(const char *s) const noexcept
 #endif
 
 	if (fold_case) {
-		return fold_case.IsIn(s);
+		return substring
+			? fold_case.IsIn(s)
+			: fold_case == s;
 	} else {
-		return value == s;
+		return substring
+			? StringFind(s, value.c_str()) != nullptr
+			: value == s;
 	}
 }