Bundle Gtk+ properly, if needed

[probonopd]

In some rare cases, it may be needed to bundle GTK+ inside the AppImage (e.g., if the application requires a newer version of GTK+ than what can be assumed to be part of all target systems). Check

https://github.com/jessevdk/gtk-mac-bundler

The script gtk-mac-bundler is a helper script that creates application bundles form GTK+ executables for Mac OS X. The resulting bundle will contain a complete self-hosting GTK+ installation, ready to run on any computer with Mac OS X 10.4 or later installed.

GTK+ and its companion libraries are automatically put into the bundle, but the application packager must tell the script what other files and directories to copy.

Also see https://github.com/NixOS/nixpkgs/issues/25838.