TODOs in this repo should point to useful github issues

[zepumph]

PhET has a policy that TODOs in production code are tied to a github issue to help in scheduling their work. This policy was created after many TODOs were created without a corresponding github issue associated with them in this repo. Never the less, this code is still production code, and TODOs should adhere to this philosophy. I'll mark TODOs in this repo to this issue now, but this should NOT be a catch all, work should be done to handle, triage, schedule and fix the work outlined by the TODOs in this repo (it is production code!).

(see work done in phetsims/tasks#1129 also)

UPDATE by @samreid, currently at:

• 315 occurrences of this issue in scenery/

[pixelzoom]

Also adding... Additional TODOs should not be created that point to this issue. And there are currently 325 TODOs that point to this issue.