[Feature Request] Introduce an AppImage build please

[pasnox]

Please add support for a testable AppImage similar to Wargus and War1gus.

[andreas-volz]

Hi pasnox,
I'd do that, but I've no idea how to do this. The build environment is different from Wargus/War1gus and Tim has to help here to answer how this was done. I've only a Linux developer system.

[andreas-volz]

Ah wait, I messed up AppImage with Apple stuff in the early morning. :-P
Hm, this is for sure a Linux task, but I've also not yet done this. I'd have to find it out....

Wargus/stratagus#416