State: Loading

### Description

Would be great for widgets to be able to support a "loading" state, where `msg.loading` can be passed as `true` or `false`, or maybe even as a number out of 100 for a progress, and then in place of any widget a loading/progress spinner shows until the state is reset.

This would follow the same architecture/approach as the `enabled` state found in https://flowforge.github.io/flowforge-nr-dashboard/contributing/guides/events.html#widget

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate