Eliminate some building issues on Windows

- Use --large-address-aware instead of /LARGEADDRESSAWARE with MinGW
- Adjust an #include in libtiled\objectgroup.cpp
- Build tmxviewer as a console application
This was necessary for console output (--help, --version), similar to e46c4d4fcb344fad99ce1e2d17a3bbeb4e24cd5f. Actually it does appear to spawn a console window when run via "run" or "Open with" rather than in a pre-existing console window... whoops. Should I remove this change then?