Pointer cleanup (#242)

- Made greater use of smart pointers, eliminating many instances of manual memory management
- Do not use pointers at all for many non-polymorphic classes
- Assorted other code cleanup

model/unit_tests/CMakeLists.txt

@@ -22,6 +22,7 @@ if (Qt6Test_FOUND)
   qt6_wrap_cpp (TestXmlLabel_moc_sources TestXmlLabel.h)
   add_executable (TestXmlLabel TestXmlLabel.cpp ${TestXmlLabel_moc_sources})
   target_link_libraries (TestXmlLabel Model Qt6::Test)
+  target_compile_definitions(TestXmlLabel PRIVATE TEST_DIR="${CMAKE_CURRENT_SOURCE_DIR}")
   add_test (NAME XmlLabel COMMAND TestXmlLabel)
 
   #=======================================