I'm going to get started on the level editor now. I'll start off simple with object selection and re-positioning using the mouse. Before I get to the editor code itself, there are a couple of things that I need to add first..
A Bullet Hell gamedev tutorial for your browser, written in JavaScript and HTML. Working from the simplest game to something a lot more complex in small, manageable steps. This isn't like a normal gamedev tutorial - you have a chance to choose the direction I take!