Adding ability to read from gzip or zip files

[a2geek]

I have a number of disk images that are in an archive, and it gets annoying to decompress them. So, after a quick bit of 'googling', I saw that it was relatively simple to identify file types.

This PR adds the ability to load a disk "still" in gzip or zip format. For the zip file, I reused the isFile*(...) methods to locate the first disk image in the archive.

After adding the code, I realized that LoadResource(...) is used in multiple locations to load any remote resource. If you think that may be a problem (or this should be restructured), please let me know!

[ivanizag]

With this implementation a compressed DSK file is writable, but writes won't work. fileNib.saveTrack() doesn't know that the file is compressed, so it writes uncompressed tracks on the zip file.

The solution may be to set d.nib.supportsWrite to false in LoadDiskette(). We would need LoadResource() to return also if the resource is writable or not.

[a2geek]

First draft setting a writeable flag. I didn't notice anything specific in the disk images (no outright error). But the changes did not persist. I am uncertain what the behavior was before, so hopefully this is the right direction!

[ivanizag]

Yes, that's it. Now the zip file is not modified.

Thanks!