AppImage generation for Linux

[christo]

 AppImage is a popular format for software bundling. I'm able to generated C::B AppImage with minimal host dependency via github action.

https://github.com/josephch/codeblocks/actions/runs/27463347269#artifacts

Github workflow : https://github.com/josephch/codeblocks/blob/custom/.github/workflows/build_appimage.yml

I think it is a good idea to distribute our releases in AppImage format also, as it can be used on many distros.

There are additional configuration options required - to use bundled software if present and to use static wxwidgets. Those are added to svn.