gui: fix string related C++20 changes

alib2gui/src/Algorithm/Algorithm.cpp

@@ -60,7 +60,8 @@ bool Algorithm::canAddOverload(const Algorithm::Overload& overload) {
 }
 std::string Algorithm::getPrettyName() const {
-    std::string eps = u8"\u03B5";
+    std::u8string str = u8"\u03B5";
+    std::string eps ( str.begin ( ), str.end ( ) );
     static std::map<std::string, std::string> prettyNames = {
         {"automaton::simplify::SingleInitialState", "Single Initial"},
         {"automaton::simplify::EpsilonRemoverIncoming", eps + "-Remove In"},

alib2gui/test-src/RegistryTest.cpp

@@ -41,6 +41,9 @@ TEST_CASE ( "Registry", "[unit][gui][registry]" ) {
 		auto* algo = Registry::getAlgorithm("automaton::transform::AutomataConcatenationEpsilonTransition");
 		REQUIRE(algo != nullptr);
-		CHECK(algo->getPrettyName() == u8"\u03B5-Concatenate");
+
+		std::u8string name = u8"\u03B5-Concatenate";
+		std::string expected ( name.begin ( ), name.end ( ) );
+		CHECK(algo->getPrettyName() == expected );
 	}
 }