Change dirty => needsUpdate?

[samreid]

From the code review in #317

    // @public {boolean} - true if the charge layout must be updated
    //REVIEW*: It's a preference, but a method marking things as needing an update has been useful.
    this.chargeLayoutDirty = true;

I can see plenty of needsUpdate in three.js, can you point to examples in PhET code? Specifically, are you recommending renaming this to chargeLayoutNeedsUpdate?

[samreid]

We discussed this, closing.