Change state of the plugin to respond to events in the editor.
This is called on meaningful events in the editor, such as change of selection
or caret position (but not simple typing of alphanumeric keys). It gives the
plugin a chance to update the CSS of its button.
For example, the "bold" plugin will highlight/unhighlight the bold button depending on whether the
characters next to the caret are bold or not.
Only makes sense when `useDefaultCommand` is true, as it calls Editor.queryCommandEnabled(`command`).