Bring back colorpicker updating

Summary:
Looks like I left this code out of the blockly refactor. :-/
This change just brings back the code.
TODO: Test Hotnumber up the wazoo

Test Plan:
- Create two fill commands in a program, make them different colors
- When you click on each, you see the appropriate color

Reviewers: brian

Reviewed By: brian

Subscribers: #cs

Differential Revision: http://phabricator.khanacademy.org/D12675
Fixes T3066 - Makes cursor usable

Test Plan:
- Write cursor(HAND); in program, see cursor change to hand
cursor() can accept 0-3 arguments

The 3 argument one doesn't really make sense for our environment,
but I'll whitelist it anyway. (It accepts PImage).

Test Plan:
Put following in program:
```
cursor(TEXT);

cursor();

noCursor();
```
Don't see an error message for any of them.

Auditors: john
Merge branch 'master' into html