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/NumberWidget.ts

@@ -64,7 +64,7 @@ export class NumberWidget extends BaseWidget implements INumericWidget {
     ctx.fill()
 
     if (show_text) {
-      if (!this.disabled) {
+      if (!this.computedDisabled) {
         ctx.stroke()
         // Draw left arrow
         ctx.fillStyle = this.text_color