DMG compression algorithm

[aonez]

As requested by akrabu139 in the forum:

• Add algorithm selector
• Add compression level/method
• Study the possibility of enabling encryption: as of Apr 24, 2020, this seems to be not possible.

[drsassafras]

I logged on to make a feature request of encrypted DMG archives, but see its already been brought up.

Ive used this app for many years and love it. Thanks for the great app!