Support associated socket for widgets (#891)

This PR is the litegraph side change necessary for widget sockets
feature in ComfyUI_frontend. Changes include

- Add readonly `Widget.computedDisabled` property for getting the
computed disabled state. When the associated socket is connected, the
widget is disabled
- Dynamically show the associated socket when
      - the mouse is over the widget
      - the slot is valid during link drop
      - the slot is connected
- Removes the legacy widget drop behavior

Ref: https://github.com/Comfy-Org/rfcs/pull/9

src/types/widgets.ts

@@ -153,24 +153,38 @@ export interface IBaseWidget {
   /**
    * The computed height of the widget. Used by customized node resize logic.
    * See scripts/domWidget.ts for more details.
+   * @readonly [Computed] This property is computed by the node.
    */
   computedHeight?: number
 
   /**
    * The starting y position of the widget after layout.
+   * @readonly [Computed] This property is computed by the node.
    */
   y: number
 
   /**
    * The y position of the widget after drawing (rendering).
+   * @readonly [Computed] This property is computed by the node.
    * @deprecated There is no longer dynamic y adjustment on rendering anymore.
    * Use {@link IBaseWidget.y} instead.
    */
   last_y?: number
 
   width?: number
+  /**
+   * Whether the widget is disabled. Disabled widgets are rendered at half opacity.
+   * See also {@link IBaseWidget.computedDisabled}.
+   */
   disabled?: boolean
 
+  /**
+   * The disabled state used for rendering based on various conditions including
+   * {@link IBaseWidget.disabled}.
+   * @readonly [Computed] This property is computed by the node.
+   */
+  computedDisabled?: boolean
+
   hidden?: boolean
   advanced?: boolean