Disable editor-centric commands in non-editor views

For all Atom commands that are intended to operate on editor views, make
it so that those commands are only available inside an editor view. For
example, we don't want editor specific commands to show up in the
command palette when we're working in the tree view.
Scope editor-centric keymaps to editor views

For keymappings that are intended to operate in editor views, make it so
that those keymappings are only active inside an editor view.
Fix variable name in 'center-previous-diff' command