linux: add support for AppImage packages

AppImage is a more flexible packaging solution for Linux than what
PyInstaller currently offers.

Some of the benefits are:

- Ability to ship the app with predefined environment variables
(solving issues like #321)
- Improved integration with the desktop environment (if you're using
go-appimage[1] or libappimage[2])
- Much lighter in size (~27 MB difference)

To actually make the AppImage, I'm using appimage-builder[3], which
makes the process a bit easier.

For more details about the AppImage technology, check out their
website[4].

[1] https://github.com/probonopd/go-appimage
[2] https://github.com/AppImageCommunity/libappimage
[3] https://appimage-builder.readthedocs.io/en/latest/index.html
[4] https://appimage.org/

.gitignore

@@ -16,3 +16,9 @@ log.txt
 /lock.file
 settings.json
 /lang/English.json
+# AppImage
+/AppDir
+/appimage-builder
+/appimage-build
+/*.AppImage
+/*.AppImage.zsync