Various backend improvements.

- Rewrite windows loader to use standard Toolhelp32 snapshots instead of
  hardcoding internal Windows structures with GetPebAddress.

- Refactor file-finding code to find_files_list in utils.h.

- Add test for existence of game_data.ini.

- Rewrite read_* functions in favor of passing void * pointers directly
  to read_raw.

- Move enumerate_vector and write_int to DFInstance from the
  platform-specific classes.

- Use more C++ casts instead of C-style casts.
Add windeployqt.sh, instructions to BUILDING.rst.

windeployqt.sh automatically traces DLL dependencies and copies the
appropriate files to the build directory for manual execution or packaging.