Build system tidying; support for easier packaging.

* Refactored compilation script downloads into a function.
* Added downloading of shareware and mindgrid data (easier to
distribute these in the bundle).
* Updated .app build to include ancillary files.
* Updated clean script and .gitignore correspondingly.
* The engine now searches for 10 possible pak files, ignoring breaks in
the sequence (makes packaging much easier).
* Scripts are quieter and it is possible to optionally not clean out
the engine code.