Update to Qt6

- New baseline minimum platform is Ubuntu 22.04
    - Qt6 requires at least 6.2
        - some deprecations may be flagged on later versions (e.g. 6.8)
    - CMake requires at least 3.22
- Include build-tests.yml github action to validate builds on mulitple platforms
- QtTest is no longer optional since it easily comes along for the ride with Qt
- Replaced QStringRef in model::SubstitutionField with simple ParserState class
- Removed deprecations up to Qt 6.2

model/Frame.cpp

@@ -60,6 +60,15 @@ namespace glabels
 		}
 
 
+		Frame::~Frame()
+		{
+			while ( !mMarkups.isEmpty() )
+			{
+				delete mMarkups.takeFirst();
+			}
+		}
+
+
 		QString Frame::id() const
 		{
 			return mId;