Convert hooks to events, deprecate hooks

[pfrenssen]

We should provide events as alternatives for our hooks, and deprecate the hooks by emitting a deprecation warning when they are implemented. It is beneficial for people porting their code from D7 to keep the hooks around for some more time. We can then remove them in our next major release.

[pfrenssen]

I had originally assigned this to the beta1 milestone but since this is an introduction of a new feature along with a deprecation of an existing feature this doesn't break BC and is fine to implement after beta1.

[MPParsley]

Great idea! See https://www.drupal.org/node/2881531