Cleanup target properties

* Add getters for surface & toolbar
* Nullify references when destroying the target
* Removing caching of dmDoc in SA target
* Bonus: Remove unnecessary local target cache in demo

Change-Id: I954e32373b12f512a4a204f1fb9e91e105dc98ff
Multi-surface fixes

* Don't ve.debounce the prototype function, otherwise deboucing
  occurs all instances, instead of per-instance.
* Check the range is from the right document in surface observer

Bonus: this.documentMousedownHandler is unnecessary duplication

Change-Id: I2f6837562182fd9cc498c25f57388a2e1e74690e
Merge "Cleanup target properties"
Merge "Multi-surface fixes"