Conditionally set editor-related events

ImageEditors don’t have save or modified events.
Use an `atom.workspace` method to destroy the open editor

ImageEditors don’t have a `destroy` method
Use pane to destroy the current editor

So that we can just `destroy` our editor without wondering if it’s the
active item or not.
Adjust test to pass in pane to preview
Add spec for `close`
Replace deprecated method call in test
Merge branch 'fix-image-files'