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;
 	}
 }

src/song/StringFilter.hxx

@@ -33,13 +33,19 @@ class StringFilter {
 	 */
 	IcuCompare fold_case;
 
+	/**
+	 * Search for substrings instead of matching the whole string?
+	 */
+	bool substring;
+
 public:
 	template<typename V>
 	StringFilter(V &&_value, bool _fold_case)
 		:value(std::forward<V>(_value)),
 		 fold_case(_fold_case
 			   ? IcuCompare(value.c_str())
-			   : IcuCompare()) {}
+			   : IcuCompare()),
+		 substring(_fold_case) {}
 
 	bool empty() const noexcept {
 		return value.empty();