Swap underlying editor correctly when calling setModel on editor element

Previously, when `setModel` was called, we forgot to update the pointer
to the component in the newly supplied editor. This was causing the
element to not update in response to model updates but only as a result
of focus or visibility changes. We suspect this regressed during the
rewrite of the editor rendering layer.

With this commit we will now correctly swap the element's underlying
editor by updating the component pointer on the newly supplied editor.
Also, if the element was already attached to another editor, we will
null out the component reference on it, because one instance of
`TextEditorElement` can only represent one instance of `TextEditor`.

src/text-editor-component.js

@@ -174,6 +174,10 @@ class TextEditorComponent {
   }
 
   update (props) {
+    if (props.model !== this.props.model) {
+      this.props.model.component = null
+      props.model.component = this
+    }
     this.props = props
     this.scheduleUpdate()
   }