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/ParserState.cpp

@@ -0,0 +1,87 @@
+/*  ParserState.cpp
+ *
+ *  Copyright (C) 2025  Jaye Evins <evins@snaught.com>
+ *
+ *  This file is part of gLabels-qt.
+ *
+ *  gLabels-qt is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  gLabels-qt is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with gLabels-qt.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+
+#include "ParserState.h"
+
+#include <algorithm>
+
+
+namespace glabels
+{
+	namespace model
+	{
+
+		ParserState::ParserState( const QString& string,
+		                          unsigned int   pos )
+			: mString(&string),
+			  mPos( pos )
+		{
+		}
+
+
+		QChar
+		ParserState::operator[]( unsigned int i ) const
+		{
+			auto ii = mPos + i;
+			if ( ii < mString->size() )
+			{
+				return (*mString)[ii];
+			}
+			else
+			{
+				return '\0';
+			}
+		}
+
+
+		bool
+		ParserState::isNextSubString( const QString& s ) const
+		{
+			for ( unsigned int i = 0; i < s.size(); i++ )
+			{
+				if ( operator[](i) != s[i] ) return false;
+			}
+			return true;
+		}
+
+
+		qsizetype
+		ParserState::pos() const
+		{
+			return mPos;
+		}
+
+
+		qsizetype
+		ParserState::charsLeft() const
+		{
+			return mString->size() - mPos;
+		}
+
+
+		void
+		ParserState::advanceChars( unsigned int i )
+		{
+			mPos = std::min( mPos + i, mString->size() );
+		}
+
+	}
+}