First rough take at Linux AppImage packaging.

[tmontes]

Contributing towards #150.

[tmontes]

PRELIMINARY RESULTS:

• Packaged latest Mu Editor master under Ubuntu 18.04:

  • Fails if no --icon-path is given.
  • For some reason, build/'Mu Editor.AppDir'/ is created -- quoted, yes. How on earth?! :)

• Run results:

  • Mu fails to create its venv if wheels are not bundled -- downloading onto a read-only dir is a no no.
  • Cannot use turtle -- looks like macOS packaged Mu fails at running tkinter / turtle #122 all over again! :)

NEXT:

• Fix the turtle thing.
• For now the .desktop file is hardcoded with Categories=Education -- maybe create an issue for that.

[tmontes]

MERGING:

• Fixed the turtle thingie.
• Logged followup issue Linux AppImage Polishing #184.
• Created this Mu PR to address the dynamic system interpreter path and avoid Mu venv re-creation on every run.