Implement interactive restarting

[alexrutar]

This PR implements interactive restarting. In order to implement this, unfortunately the type parameters T and R had to be added to the Event enum, since the Event enum now must carry a variant which can used to send an Injector<T, R> back to the Event source. This means type inference fails a bit more easily now, and adds some boilerplate to type annotations in certain situations. However, overall, having the type parameters T and R now accessible to the Event enum means adding new variants in the future involving the renderer or the picker item type will be non-breaking.

The interactive restarting was otherwise quite simple to implement inside the picker (just restart, and send a new injector down the channel). However incorporating it into an application can be a bit more work. Therefore I added reasonably comprehensive examples restart and restart_ext which demonstrate how this might be done in practice and why the API has the shape that it does.