Attach, show and hide pane item views with native DOM APIs

We continue to invoke SpacePen attach hooks on any shim wrappers
for backward compatibility, but SpacePen wrappers are no longer required
for attoch/hide/show as they were previously. Next: removal.
Use native DOM APIs to remove pane item views
Merge pull request #3721 from atom/ns-pure-custom-element-pane-items

Allow pure custom element pane item views