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/widgets/ButtonWidget.ts

@@ -45,7 +45,7 @@ export class ButtonWidget extends BaseWidget implements IButtonWidget {
     ctx.fillRect(margin, y, width - margin * 2, height)
 
     // Draw button outline if not disabled
-    if (show_text && !this.disabled) {
+    if (show_text && !this.computedDisabled) {
       ctx.strokeStyle = this.outline_color
       ctx.strokeRect(margin, y, width - margin * 2, height)
     }