Previously, when calling `destroy` on a `PanelContainer` containing
multiple panels, Atom would throw a `Cannot read property 'destroy' of
undefined` exception. This was due to iterating over the panels while at
the same time destroying them, which caused the iterated array to be
modified during the loop.
With this commit we slice the array before iterating over it so that
destroying a `PanelContainer` doesn't throw exceptions anymore.
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.
