Trash Can for Design Mode [2.0]

Drag and drop it in.

Very cool! Useful for when we just need to get rid of lots of random stuff and it becomes a pain to right click and delete all of it.

The trash can feature is now fully complete, in the nightlies. If you've been trying it out already, we've made a few tweaks to it since then.

- A visual cue on the blocks you're dragging in that they will be deleted.
- A tooltip to convey to new users what to do.
- A smaller icon.
- Better hit detection (intersection vs. mouse position)