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/BaseWidget.ts

@@ -23,6 +23,7 @@ export abstract class BaseWidget implements IBaseWidget {
   last_y?: number
   width?: number
   disabled?: boolean
+  computedDisabled?: boolean
   hidden?: boolean
   advanced?: boolean
   tooltip?: string