song/Filter: add operator "contains"

Closes #410

src/song/Filter.cxx

@@ -206,6 +206,20 @@ ExpectQuoted(const char *&s)
 static StringFilter
 ParseStringFilter(const char *&s, bool fold_case)
 {
+	if (auto after_contains = StringAfterPrefixIgnoreCase(s, "contains ")) {
+		s = StripLeft(after_contains);
+		auto value = ExpectQuoted(s);
+		return StringFilter(std::move(value),
+				    fold_case, true, false);
+	}
+
+	if (auto after_not_contains = StringAfterPrefixIgnoreCase(s, "!contains ")) {
+		s = StripLeft(after_not_contains);
+		auto value = ExpectQuoted(s);
+		return StringFilter(std::move(value),
+				    fold_case, true, true);
+	}
+
 	bool negated = false;
 
 #ifdef HAVE_PCRE

src/song/StringFilter.hxx

@@ -96,7 +96,9 @@ public:
 	const char *GetOperator() const noexcept {
 		return IsRegex()
 			? (negated ? "!~" : "=~")
-			: (negated ? "!=" : "==");
+			: (substring
+			   ? (negated ? "!contains" : "contains")
+			   : (negated ? "!=" : "=="));
 	}
 
 	gcc_pure