Refactoring with a MapStore

A great deal of the complexity of the current code is that we must chain
2 reactive values (one in the map "GameScene.MapPlayersByKey" and one in
the snapshot store).

The new generic MapStore class can be used to listen to stores inside a map.
When the store inside the map, or the map itself is modified, the
resulting store is updated.

front/src/Stores/PlayersStore.ts

@@ -12,7 +12,7 @@ let idCount = 0;
 function createPlayersStore() {
     let players = new Map<number, PlayerInterface>();
 
-    const { subscribe, set, update } = writable(players);
+    const { subscribe, set, update } = writable<Map<number, PlayerInterface>>(players);
 
     return {
         subscribe,