network: Make it a template class

The current Network panel class relies on GtkBuilder
when it could use a more modern feature that is the
template class.

By making it a template class, not only the Network
panel is slightly more performant, but it's also
simpler and easier to read.

This commit, then, turns the Network panel into a
template class, and cleans up the code to make it
work.

https://bugzilla.gnome.org/show_bug.cgi?id=785581