Ability to insert elements into the canvas

[crcn]

trafficstars

Part of #839

Engine TODOs:

• [ ] display slot placeholders for {slot}
• [ ] for elements that are in slots, need to add info about that in sourceInfo

UI TODOs:

• [ ] Toolbar containing
  • [ ] zoom level
  • [ ] insert text button
  • [ ] insert element button
  • [ ] quick search button
• [ ] typing T should allow for text to be inserted
• [ ] should be able to edit text in the canvas
• [ ] should be able to drag elements around
• [ ] typing E should allow for elements to be inserted

[crcn]

This requires CRDTs