Description: Improve the message list to deliver an awesome user experience
While ListView is not too bad at handling a gazillion of list items our current adapter makes things difficult by always loading all messages (not completely, but still) into memory. Optimizing that is the most important part of this project.
Implement pull-to-refresh to start checking mail
Add section headers to visually separate messages by sort key (usually the date)
Populate the unified inbox temporalily evenly between folders (Issue 4086)