Firejail support

[parcelcat]

Adding Firejail support to appimage-manager would allow managed AppImages to be run in a sandbox, which enhances security. This feature has been implemented in appimaged at AppImageCommunity/appimaged#41. It would be nice to see appimage-manager have feature parity.

[azubieta]

Doing an effective sandboxing of an application is a tricky issue, and must be performed for each application. Firejail provides a generic recipe which will definitively fall short for many apps.

The big issue here is who should we trust to create such recipe. The application author? A third party?

Making the desktop files to use firejail when launching the applications is simple but it will not be a big improvement in terms of security as long as the above mentioned issues is not solved.