First release of QUCS AppImage

This is a QUCS 0.0.20-rc2 (don't get scared for the number, it works quite well).
It has no internal documentation nor examples (to keep the image small).

To use it, just download the AppImage, make it executable, and run it.

I checked in Ubuntu 16.04, 18.04, and 20.04 and it seems to work ok.