webadmin: Added method AddChild() to SystemTreeItemModel

This method adds a child to a node, and sets the child node's parent
correctly, changed SystemTreeModel code to use this to build the system
tree, and turned the setParent() method to private. This is much more
object-oriented than having developers maintain the proper structure of
the tree themselves.

Previously this change would have broken functionality, but following
patches [Ie4341a4c4e1740c075987e2a57c0844fa72b971e] and
[I79f2e9f18e08b89cdfe84d4657bce1de8f8988bb] it should be okay.

Change-Id: I000cd0513bb725ae77a33cefeeea49fb66e3b7fc
Signed-off-by: Lior Vernia <lvernia@redhat.com>