Network manager glib

Created by: SunilMohanAdapa

This fixes #124 (closed). This pull request makes Plinth use libnm (introduced with network-manager 1.0) with gobject-introspection instead of python-networkmanager. The reasons have been highlighted in original issue. Since all the required dependencies are now avialable, this pull request also enables networks modules.

There is one known issue: trying to retrieve ip_address during edit connection results in a segfault. This is most likely an issue with python3-gi or libnm. So, for now, showing earlier set IP address has been disabled. Setting a new one works. I believe we can still merge this with this small issue present.