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/Model.h

@@ -233,11 +233,11 @@ namespace glabels
 			// Private data
 			/////////////////////////////////
 		private:
-			int                       mUntitledInstance;
-			bool                      mModified;
+			int                       mUntitledInstance{0};
+			bool                      mModified{true};
 			QString                   mFileName;
 			Template                  mTmplate;
-			bool                      mRotate;
+			bool                      mRotate{false};
 
 			QList<ModelObject*>       mObjectList;