Class globals multiplayer model

Autonomous singleton class that monitors multiplayer aircraft,
maintains data in various structures, and raises signal
"/sim/signals/multiplayer-updated" whenever an aircraft
joined or left. Available data containers are:

multiplayer.model.data:        hash, key := /ai/models/~ path
multiplayer.model.callsign     hash, key := callsign
multiplayer.model.list         vector, sorted alphabetically (ASCII, case insensitive)

All of them contain hash entries of this form:

{
callsign: "BiMaus",
path: "Aircraft/bo105/Models/bo105.xml",      # relative file path
root: "/ai/models/multiplayer[4]",            # root property
node: {...},        # root property as props.Node hash
model: "bo105",     # model name (extracted from path)
sort: "bimaus",     # callsign in lower case (for sorting)
}