Appimage Distribution

[Ryex]

Summary:

Adds a build-appimage command to the setup.py that builds off the existing build-debian command to package the source and then writes the appropriate metadata files to a dist_appimage directory.

requires python-appimage package to be installed via pip to succeed.

Related Issue(s):

#1091

Reviewer's Checklist:

• The header of all files contain a reference to the repository license
• The overall test coverage is increased or remains the same as before
• All tests are passing
• All flake8 checks are passing and the style guide is followed
• Documentation (as docstrings) is complete and understandable
• Only files that have been actively changed are committed

[vkbo]

Thanks a lot for contributing this. I made a few minor changes to the setup function, and tested it. I had no problem building it and running it.

Just a FYI: I reverted the change of spelling. Everything in novelWriter is UK English. That's the version I learned :)