Handle Gtk+ better

Currently we are bundling `libgtk-3.so.0` and `libgdk-3.so.0` but not the resources (e.g., themes) that go with them.

This results in


when it should be


```
(AppRun:7980): Gtk-WARNING **: 11:40:02.419: Theme parsing error: gtk-contained.css:4322:148: Missing name of pseudo-class

(AppRun:7980): Gtk-WARNING **: 11:40:02.419: Theme parsing error: gtk-contained.css:4367:14: 'min-height' is not a valid property name

(AppRun:7980): Gtk-WARNING **: 11:40:02.420: Theme parsing error: gtk-contained.css:4587:23: No property named '-gtk-icon-transform'
```

We should either __not__ bundle `libgtk-3.so.0` and `libgdk-3.so.0`, or __also__ bundle the resources (e.g., themes) that go with them.

With something like `GTK_EXE_PREFIX=squashfs-root/usr/ GTK_THEME=Adwaita squashfs-root/AppRun` it may be possible to load a bundled Gtk+ theme rather than the system's one, which is broken for us most of the time.

More information:
https://developer.gnome.org/gtk3/stable/gtk-running.html