Send request for addon list on client connect.

Signed-off-by: Chris Sanders <zidane@outlook.com>
Implement DefaultedMap/Key system

Heavily based on work done in the Spout engine to send serialized data
over the network in easy to use key formats. This is the groundwork of
establishing database-like storage behind concepts such as the GUI.

Signed-off-by: Chris Sanders <zidane@outlook.com>