Interface for classes implementing UI panels.
| contextType | Describes in which context a VisualElement hierarchy is being ran. |
| contextualMenuManager | The Contextual menu manager for the panel. |
| dispatcher | This Panel EventDispatcher. |
| focusController | Return the focus controller for this panel. |
| isDirty | Checks whether any element within the panel has had any changes to its state since the panel was last rendered. |
| visualTree | Root of the VisualElement hierarchy. |