Make 'core:close' destroy the pane itself if it is empty

src/workspace.coffee

@@ -132,6 +132,10 @@ class Workspace extends Model
   destroyActivePaneItem: ->
     @activePane?.destroyActiveItem()
 
+  # Public: destroy/close the active pane.
+  destroyActivePane: ->
+    @activePane?.destroy()
+
   # Private: Removes the item's uri from the list of potential items to reopen.
   itemOpened: (item) ->
     if uri = item.getUri?()