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
