Remove view registry references from the workspace model layer

Removed various ::getView methods from the model. Using the atom.views
global in the views for now, but going to switch them over to using a
locally assigned view registry instead in a subsequent commit.
Change ::setModel to ::initialize on all element classes
Simplify ViewRegistry::addViewProvider API

It now takes two arguments, a model constructor and a create view
callback that is passed the model.

Signed-off-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
Merge pull request #4365 from atom/ns-view-registry-cleanup

Clean up view registry API