Convert props package to an internal package

[Smithor]

I'm not sure if it's worth bothering to make get/setPropValue and initProps abstract and have a DefaultPropObject for some of the same reasons as ViewController. I almost consider PropObject an internal class to give View and friends easy property handling, archival and undo.

Originally posted by @reportmill in #15

Right now, the props package API is public and a part of every single view or aspect that could be touched by a SnapKit user. It should have its footprint reduced as much as possible to better suit its role as an internal package. Some possible steps that could be included are:

• Change method visibility from 'Public' to 'protected' or default as appropriate.
• Modularize SnapKit and make props package open only to other module members of the SnapKit library.

This process will probably be a larger one, with more details found as work continues. It is important to simplify the inherited API of the Views, since they involve functionality that the user should not need to interact with on a regular basis.