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/ModelImageObject.cpp

@@ -316,7 +316,7 @@ namespace glabels
 				}
 
 				mImage = new QImage(value);
-				quint16 cs = qChecksum( (const char*)mImage->constBits(), mImage->sizeInBytes() );
+				quint16 cs = qChecksum( QByteArray( (const char*)mImage->constBits(), mImage->sizeInBytes() ) );
 				mFilenameNode = TextNode( false, QString("%image_%1%").arg( cs ) );
 
 				emit changed();