Editor: Doodad Properties Window

The Doodad Properties window brings many features that used to be
available only in the `doodad` CLI tool into the Doodad Editor.

* In the Doodad Editor there is a new menubar item: "Doodad" which
  corresponds to the "Level" menu when you're editing a level.
* The "Doodad" menu has two items:
  - "Doodad Properties" (NEW)
  - "Layers" (moved here from the Tools menu)
* The Doodad Properties window lets you edit the Title and Author values
  of the doodad, as well as modify its Tags and manage its Script.
* Its script can be attached (browse for .js file on disk), its existing
  script saved back to disk (dev shell prompt) or deleted altogether
  from the doodad.
* You can create, modify, and delete Tags on the doodad.

Other changes:

* In the Level Editor, the "Level->Page Settings" menu is renamed to
  "Level->Level Properties" to match with "Doodad->Doodad Properties"
  and the pop-up window is retitled accordingly.
* The Exit Flag only exits if the Player touches it - not just any
  mobile doodad!

dev-assets/doodads/objects/exit-flag.js

@@ -7,6 +7,11 @@ function main() {
 			return;
 		}
 
+		// Only care if it's the player.
+		if (!e.Actor.IsPlayer()) {
+			return;
+		}
+
 		if (e.InHitbox) {
 			EndLevel();
 		}