Cleanup Network panel

This is a somewhat lengthy but easy cleanup. There are 4 important aspects of this merge request:

1. It removes all usages of the deprecated g_type_class_add_private from the Network panel;
2. It ports the Net* objects to G_DECLARE_{DERIVABLE|FINAL}_TYPE(), removing the old style GObject boilerplate;
3. Final classes had their private struct unified with their regular struct;
4. At last, #pragma once was used in the headers

With that, we have a net loss of 192 lines of code:

 panels/network/cc-network-panel.h    |   6 +-----
 panels/network/cc-wifi-panel.h       |   6 +-----
 panels/network/net-device-ethernet.c |  16 ++++++++++++++++
 panels/network/net-device-ethernet.h |  39 ++-------------------------------------
 panels/network/net-device-mobile.c   | 144 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++----------------------------------------------------------------------------
 panels/network/net-device-mobile.h   |  31 +++----------------------------
 panels/network/net-device-simple.c   |  42 +++++++++++++++++++++---------------------
 panels/network/net-device-simple.h   |  26 +++-----------------------
 panels/network/net-device-wifi.c     | 150 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++------------------------------------------------------------------------------
 panels/network/net-device-wifi.h     |  29 +++--------------------------
 panels/network/net-device.c          |  28 ++++++++++++++--------------
 panels/network/net-device.h          |  22 ++--------------------
 panels/network/net-object.c          |  67 ++++++++++++++++++++++++++++++++++++++++++-------------------------
 panels/network/net-object.h          |  23 ++---------------------
 panels/network/net-proxy.c           | 100 +++++++++++++++++++++++++++++++++++++++++++++++++---------------------------------------------------
 panels/network/net-proxy.h           |  30 +++---------------------------
 panels/network/net-vpn.c             |  81 +++++++++++++++++++++++++++++++++++++--------------------------------------------
 panels/network/net-vpn.h             |  30 +++---------------------------
 panels/network/network-dialogs.h     |   5 +----
 panels/network/panel-common.h        |   5 +----

20 files changed, 344 insertions(+), 536 deletions(-)

Please follow the commit order when reviewing this merge request. Staring at the raw diff won't help the review process. The commits are already obvious and self-contained, and they should substantially ease the review.

@jonathankang @robert.ancell