AppImage support

[danirod]

This PR adds a script that builds the AppImage for Cartero.

To use the script, use it as follows:

# To build in default mode:
build-aux/appimage-build.sh stable

# To build in development mode:
build-aux/appimage-build.sh devel

Additionally, this commit adds a debug GitHub Action that will run the script using Ubuntu 24.04, which is the minimum version of Ubuntu that provides the required version of libgtk-4-dev and libadwaita-1-dev.

The generated AppImages will require at least glibc 2.38, since they are built by an Ubuntu 24.04 runner. Therefore, Debian 12, you are probably out of the equation at the moment until someone figures out how to compile in Debian 12 or Ubuntu 22.04 using a newer version of libgtk-4-dev and libadwaita-1-dev than the one provided by the system.

The appimage.yaml action is not meant to be merged. Once it works, each job has to be integrated into the nightly.yaml and stable.yaml action.

🙅 QA / Stoppers:

• KDE: Cartero icon is rendering with black background in the taskbar.
• All platforms: icons are failing, in general. Maybe I need to vendor the entire Adwaita icon theme, just like I do on Windows and macOS.
• Fedora 41: it seems that the output response when making an HTTP response is not coloured, maybe the GtkSourceView datafiles have to be vendored as well.