DEV: adds discourse:focus-changed app event (PR #8123)


You’ve signed the CLA, jjaffeux. Thank you! This pull request is ready for review.

@eviltrout note I couldn’t find where is this notify: false used, do you know ?

This is a walk down memory lane:

We used to put (*) topic title for certain cases, something that we totally stopped doing.

Merging this in!