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/TextWidget.ts

@@ -47,7 +47,7 @@ export class TextWidget extends BaseWidget implements IStringWidget {
     ctx.fill()
 
     if (show_text) {
-      if (!this.disabled) ctx.stroke()
+      if (!this.computedDisabled) ctx.stroke()
       ctx.save()
       ctx.beginPath()
       ctx.rect(margin, y, width - margin * 2, height)