Add titleNode to labScreen.view.initialValuesPanel

[arouinfar]

For #244

This is labScreen.view.initialValuesPanel

We'd like to instrument the "Initial Values" title and give it a visibleProperty and textProperty. The tree structure would look like this:

initialValuesPanel
    visibleProperty
  + angleReadout
  + heightReadout
  - titleNode
       textProperty
       visibleProperty
  + velocityReadout

[zepumph]

It seems like most of this happened two days after opening this issue in b703492, as part of #219, but I don't see a paper trail on that structure over there. Added a titleNode, but that is all.

Anyways. . . .

Please review in master, the readout textProperties are phetioReadOnly: true, is that correct?

[arouinfar]

The new titleNode looks good in master.

the readout textProperties are phetioReadOnly: true, is that correct?

Yes, that's correct. The readout text should match the actual data in the sim.