* Stores per-message metadata
* Fix a number of todo's in MessageList
* Clicking on checkbox toggles state, updates checked list
* clicking on star toggles favorite state
To do: When one or more messages are checked, we should slide in a
panel with action buttons.
* Attachment icon is displayed when message.mAttachment != 0
* Favorite icon is set/cleared from message but not clickable
* Selection icon is not clickable
Further progress requires implementing a custom child view which can
handle sub-clicks properly.
* Add MessageList Activity class and Manifest entry
* Add appropriate layouts, views, etc.
* Wire into FolderMessageList
Lots to do, but this gives us a useable screen.
