Implement "ask" and "answer" blocks

[tmickel]

These are currently unimplemented. Depends first on scratchfoundation/scratch-render#18 (since the "ask" happens in a say bubble), implementing the ask GUI (could feasibly be done outside the stage with floating HTML), and #287.

(Update from Paul, exactly one year later)
I wanted to add a few functionality notes here:

• There are actually two different ways ask appears: one for hidden sprites and one for visible sprites. If a sprite is hidden at the time the "ask" block starts, the question is actually put above the answer input box like a label. If it is visible at the time of "ask", it goes in modified say bubble, this is the usual way.
  
  
• Switching sprite visibility while the ask is running (it blocks) doesn't change the label mode. That is, if the question is in a say bubble when the sprite is hidden, it doesn't transfer to the answer field, it just gets hidden. Similarly the other way around. I'm not sure if we want to maintain this behavior?
  
• If the sprite is visible, the question bubble interacts with existing say/think bubble by clearing it, just like a regular "say" clears other "say". But because of the way show/hide interacts with ask above, you can actually show both a question and a different say bubble like this:
  

Given these different modes/interactions, and because say/think is implemented completely in the looks blocks and ask/answer is in sensing, one way to start would be implementing the "hidden" sprite mode, where the question is in the answer field. That would be the fastest path to getting something out. Then we could add the bubble ask version on top of that. That is just one idea though.