Make AppImage self-updateable or updateable

[leogallego]

AppImage packages are not updateable or upgradeable.

To avoid having to download and replace the AppImage every time there is an update for electronim, the appimage format supports a self-update feature that could be enabled, or as an alternative, make it updateable with 3rd party tools (AppImageUpdate).

[manusa]

Hi Leo, this sounds very interesting. I need to investigate how this feature fits in with the electron-packager+electron-builder