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/NodeSlot.ts

@@ -214,7 +214,8 @@ export abstract class NodeSlot implements INodeSlot {
     if (!lowQuality && doStroke) ctx.stroke()
 
     // render slot label
-    if (!lowQuality) {
+    const hideLabel = lowQuality || isWidgetInputSlot(this)
+    if (!hideLabel) {
       const text = this.renderingLabel
       if (text) {
         // TODO: Finish impl.  Highlight text on mouseover unless we're connecting links.