Add AppImage release building

[SpeeQz1]

Description
It would be very nice to have for the GitHub an AppImage release for Pinta especially for the 2.2 dev build. Apps like Gear Lever make AppImage integration quite easy although the compression algorithm should be considered as I believe AppImageLauncher is not maintained actively and newer AppImages built with zstd may fail.

FreeCAD AppImage issues

[JGCarroll]

Are there many libadwaita AppImages already?

Whilst I think Pinta would usually be a good candidate for being an AppImage, AppImages have to be built on the oldest stable distribution possible. Focusing on Ubuntu, Pinta would build on Ubuntu 24.04 but not 22.04; any AppImages would be implicitly only supported on 24.04+; and the GTK4 features in use are pretty new, I don't think Pinta 2.2/GTK4 would build on Debian Stable as is.

The result would likely be something most people expect to "run anywhere" but that doesn't run in most places.