Standalone macOS app

[TheLastProject]

Currently, macOS support is a bit lacking. As described in the README, py2app builds are currently aliased because the standalone build does not work. We need a way to make a proper stand-alone .app that people can download and install as-is, without having to keep the downloaded files or so around.

The current issue for standalone builds is documented on StackOverflow, but nobody ever answered and no solution was found: https://stackoverflow.com/questions/45364693/py2app-ignoring-local-packages.

Bonus points for shipping all dependencies in the .app as well!

It is okay to switch away from py2app to something else if it works better, as long as it's freely licensed.

[TheLastProject]

This seems to be working in the macos branch, just need to figure out how to use sha256sum on Travis.

[TheLastProject]

This is pretty much done, just need to be careful to not get in the way of #31.

[TheLastProject]

Scratch my previous notes, py2app seems to cause a lot of weird, small issues that make the .app version unreliable. Help is likely needed here after all.

[TheLastProject]

Currently broken in, and only in, the .app version:
https://github.com/Pext/Pext_module_pass: "Failed to load module pass from pext_module_pass. No module named 'pty'.
https://github.com/Pext/Pext_module_ssh: Infinitely stuck on loading.
A random profile named sn_0_XXXXXX, with XXXXXX being 6 digits is created each launch (what the actual hell?) [Edit: solved in 916b8a8]

Everything else seems to work.

[TheLastProject]

I believe this is fixed.

@Pext/macostesters (and others who read this), please help test https://transfer.sh/mju6q/Pext.app.zip. If it works, this'll become the official macOS .app and will be automatically built for each commit. It seems to work on my macbook but I want to be surer it's not just my device.

[TheLastProject]

I'm letting the master branch build continuous builds now, because it worked on my machine. I may remove them and reopen this if we get issues after all.