Ship AppImage of plugin that includes the dependencies

[TheAssassin]

The gtk plugin has some dependencies which we should bundle, most notably patchelf and pkgconf. This way, we don't need to have users install random versions of the tools (mostly important for patchelf).

For patchelf, an alternative solution would be to have linuxdeploy set an environment variable to its own patchelf binary that the plugin reads and uses if available.

[TheTumultuousUnicornOfDarkness]

For pkgconf, I am not sure if bundling this tool is a great idea. If we bundle pkgconf with this plugin inside an AppImage, I am afraid that we may face the same issue as described here.
IMHO, pkgconf must be present on the system, as some .pc reside on host's filesystem.

[TheAssassin]

The tool pkgconf will read the system .pc files anyway, though. I can't imagine that there's a lot of change to the way the tool works, honestly.

pkgconf is a developer tool, yes, but the dependency is also there in build environments etc. I think it's way more common than patchelf, though.