Snap packages: Add python bindings for gobject introspection

Python bindings for GObject based GTK libraries are needed by Inkscape extensions with custom GUIs. Inkscape packages for Windows, MacOS and Linux AppImages already provide these bindings. They should be also included in the Inkscape snap packages so extensions with custom GUI do not break in such environments.

Personally, I am unable to build the snap packages (I am on a VM and cannot use multipass, lxd approach fails). Hence, I list the required packages here in the hope that a more experienced developer who runs Linux natively can build the packages accordingly:

Required packages (usually installed by sudo apt install)

• python3-gi
• python3-gi-cairo
• gir1.2-gtk-3.0

Test of gi in the sandboxed snap environment

• Launch python interpreter
• Execute the following commands, there should not occur any errors:

  import gi
  gi.require_version("Gtk", "3.0")
  from gi.repository import Gtk
  from gi.repository import Gdk, GdkPixbuf 

Thanks!