Initial Doodad JavaScript System

* Add the JavaScript system for Doodads to run their scripts in levels,
  and wire initial OnCollide() handler support.
* CLI: Add a `doodad install-script` command to the doodad tool.
  * Usage: `doodad install-script <index.js> <filename.doodad>`
* Add dev-assets folder for storing source files for the official
  default doodads, sprites, levels, etc. and for now add a JavaScript
  for the first test doodad.

dev-assets/doodads/test/index.js

@@ -0,0 +1,13 @@
+// Test Doodad Script
+function main() {
+	console.log("I am actor ID " + Self.ID());
+
+	// Set our doodad's background color to pink. It will be turned
+	// red whenever something collides with us.
+	Self.Canvas.SetBackground(RGBA(255, 153, 255, 153));
+
+	Events.OnCollide( function(e) {
+		console.log("Collided with something!");
+		Self.Canvas.SetBackground(RGBA(255, 0, 0, 153));
+	});
+}