Adds search bar for adding thangs in level editor

Right now, while creating/editing levels, adding new thangs is a little painful, especially when you aren't familiar with the images for thangs, which is true for most new users. So, I've added a little search bar in the Add Thangs column on the right.

I had to create a subview for the Add Thangs column, since partially updating a view in Backbone is a little tricky, without re-rendering the complete view. I'm mostly new to Backbone, so I'm a little unsure about this.