Inventory System for Level Actors

* Added an inventory system for actors as a replacement to the arbitrary
  key/value data store. Colored keys now add themselves to the player's
  inventory, and colored doors check the inventory.
* Inventory is a map[string]int between doodad filenames
  (red-key.doodad) and quantity (0 for key items/unlimited qty).
* API methods to add and remove inventory.
* Items HUD appears in Play Mode in lower-left corner showing doodad
  sprites of all the items in the Player's inventory.

dev-assets/doodads/doors/colored-door.js

@@ -1,6 +1,7 @@
 
 function main() {
 	var color = Self.Doodad.Tag("color");
+	var keyname = "key-" + color + ".doodad";
 
 	// Layers in the doodad image.
 	var layer = {
@@ -34,8 +35,9 @@ function main() {
 				return;
 			}
 
-			var data = e.Actor.GetData("key:" + color);
-			if (data === "") {
+			// Do they have our key?
+			var hasKey = e.Actor.HasItem(keyname) >= 0;
+			if (!hasKey) {
 				// Door is locked.
 				return false;
 			}

dev-assets/doodads/doors/keys.js

@@ -2,7 +2,9 @@ function main() {
 	var color = Self.Doodad.Tag("color");
 
 	Events.OnCollide(function(e) {
-		e.Actor.SetData("key:" + color, "true");
-		Self.Destroy();
+		if (e.Settled) {
+			e.Actor.AddItem(Self.Doodad.Filename, 0);
+			Self.Destroy();
+		}
 	})
 }