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

glabels/ColorPaletteItem.h

@@ -67,7 +67,7 @@ namespace glabels
 		/////////////////////////////////
 	protected:
 		void paintEvent( QPaintEvent* event ) override;
-		void enterEvent( QEvent* event ) override;
+		void enterEvent( QEnterEvent* event ) override;
 		void leaveEvent( QEvent* event ) override;
 		void mousePressEvent( QMouseEvent* event ) override;