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/ParserState.cpp

@@ -73,7 +73,7 @@ namespace glabels
 		qsizetype
 		ParserState::charsLeft() const
 		{
-			return mString->size() - mPos;
+			return std::max( mString->size() - mPos, qsizetype(0) );
 		}