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/PictureStore.ts

@@ -0,0 +1,6 @@
+import type { Readable } from "svelte/store";
+
+/**
+ * A store that contains the player/companion avatar picture
+ */
+export type PictureStore = Readable<string | undefined>;