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/Markup.h

@@ -35,6 +35,8 @@ namespace glabels
 		class Markup
 		{
 		public:
+			virtual ~Markup() = default;
+
 			virtual Markup* dup() const = 0;
 
 			virtual QPainterPath path( const Frame* frame ) const;