Support for AppImage

[machitgarha]

Summary

Provide AppImage distribution bundle. There are ready-to-be-used scripts to generate one.

I recommend to use this one, and it's updated. Personally, I've been using it for almost (more than?) a year, and almost everything works fine (e.g. one problem is the dev tools not opening, not sure if it's related to AppImage itself or not).

Motivation

Read the description of this PR please.

Describe alternatives you've considered

As I'm using Fedora Linux, an rpm package is an alternative. However, it doesn't support auto-updates (not to mention incremental ones), and not possible to use it on some other distros.

Additional context

Nothing.