Add title and tooltip to diff preferences dropdown

This is an actionable insight from the UX Department: MRs experience async critique research.

What

The user preferences dropdown in the Changes tab doesn’t have a tooltip or title to describe its purpose.

Why

Users have to open the dropdown to understand its purpose. It’s not clear what it does just by looking at the icon.

How

Provide a View preferences tooltip so that users can understand what the icon dropdown means, without having to open the dropdown. Adding a tooltip to icons without labels is a Pajamas guideline.

Also add a View preferences title/header to the dropdown to help users understand its purpose while it’s open.