Create a pane system for pop ups and dialogs

[jessegreenberg]

Mostly in support of #359 (dialogs), but can be used for other things too. For that issue I made a prototype for managing dialogs in a "pane" system. This is as far as I got

PaneLayer: A layer that manages node siblings. Moves nodes to the front depending on user interaction, manages addition and removal of nodes. It is the client's responsibility to add the layer to the correct place in the scene graph.
topPane: A singleton type with one PaneLayer. Meant to always be on top. Is a singleton so that it is easy to add items to the top of the view through something like topPane.addToFront( node ).

I am not wed to the terminology and type names, but is just for brainstorming.

Sim.js was responsible for adding the topPane layer to the scene graph. Otherwise, a PaneLayer can be added anywhere.

Pretty simple, but I will commit to a branch and assign to @andrea-phet, it may be a stepping stone as she continues work on Dialogs.

[pixelzoom]

Reassigning to @ariel-phet for prioritization and assignment.

[ariel-phet]

This work is not currently a priority for the project, and it is unclear when it will be picked up again. Marking deferred.