network: Add connections and devices to different stack

The current Network panel is composed of a single stack and
a treeview to select the currently visible stack page. Each
stack page represents a connection or device.

The new Network panel, however, has none of the concept of
selectable pages. In the new layout, all connections and
devices appear all at once in a more compact and simpler
fashion.

This commit, then, starts moving towards a unified, pageless
panel by adding all the connections and devices to different
stacks. These different stacks are transient to the network
object, and are added at appropriate boxes, giving the panel
a unified layout.

This has some serious implications in the design of the
current code. Most of the code removals were related to the
treeview and different pages handling. No more tree model
madness is present, and the devices are now stored in a
plain simple GPtrArray.

After this patch, NetObject:add_to_stack isn't a good code
design choice anymore. This will be addressed in a future
patch.

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