Add AppImage builds

[ghost]

Setup Travis CI for build AppImage (portable binary package for Linux)

Please, setup Travis CI for auto-build AppImage (portable package for Linux) for each commit:

• AppImage Docs - https://docs.appimage.org/

TODO

0. Create ../blob/master/.travis.yml
1. https://github.com/appimage/AppImageKit
   1.1. https://github.com/probonopd/linuxdeployqt
   1.2. https://github.com/linuxdeploy
2. https://github.com/probonopd/uploadtool
3. Enable Travis CI pushing to GitHub releases for this repo.

For example, take a look on LeoCAD .travis.yml implementation:

• https://github.com/leozide/leocad/blob/master/.travis.yml

And here is how resulted builds would look like:

• https://github.com/leozide/leocad/releases/tag/continuous

For any AppImage packaging questions & support:

• https://docs.appimage.org/user-guide/faq.html#question-where-do-i-get-support

[hsanjuan]

Would you like to take a stab to the script?

It might be better to use github actions, it might facilitate the upload-to-release part, but not sure.

[ghost]

upload-to-release part

uploadtool script could did it:

• https://github.com/probonopd/uploadtool