Identify and add group of reviewers to changes to public API

The header review process that starts during release preparations has become a significant burden for people involved.

Identifying changes that will be in scope for that process immediately, and inviting relevant contributors to have a look right away, should reduce the amount of work required at the end of the development cycle, thus making it less frustrating and time consuming: changes will be fresh in the authors mind, no new code will be written based on the suboptimal API addition, etc.

A gerrit bot could identify changes that touch public header files (any header not ending with _p.h), and flag those changes so that a header review can start immediately. That header review could be started as part of regular code review; but it doesn't have to be blocking the usual review and staging process; it could instead be triggered when the change merged.

To flag such changes for header review, a gerrit bot could

• add a group (of people that are typically active in header review) to the list of reviewers
• set a topic (might be problematic if the change already has a topic)
• add a hashtag
• create a separate change with the diff of the respective header file against the previous version of Qt, using the existing script (which then ignores boring changes etc)
• create a Jira ticket (not ideal; we don't need more tickets)
• Add a custom label to the change in gerrit "API-Change"