Provide users with the ability to subscribe to keyboard and mouse events on a specific container

[mum4k]

I.e based on which container is focused.

Currently when user clicks on a container and performs a keyboard or a mouse action, only the widget in the container receives the events.

We could enhance this so that users of termdash could register additional event subscribers per focused container. This would allow them to process and act on these events outside of the widget code.