When adding object, change “add object” button to “cancel adding object” button

[nathanielrindlaub]

Right now, if you start adding an object but want to abandon it, you click off of the bbox after you've drawn it. I'm not sure how intuitive that is, so it would be helpful to have a button that allows users to exit adding object mode.

Maybe change the button when state isDrawingBbox === true? Or maybe when the focused object's object.isBeingAdded === true?

Also, work on better text than "cancel adding object"...