We lost the reporting of connection errors in the UI. These were
originally displayed in FolderMessageList, under the name of each folder.
In the new implementation, we borrow the "undo" banner from Gmail and
display it whenever there is a connection error in a mailbox.
* Add banner, and code to animate it on/off the top of the list.
* Toggle banner whenever a connection state change occurs in MessageList
* Toggle banner whenever a connection state change occurs in MailboxList
* Slight change to callback semantics for sendMailCallback
* Toggle small pane with multi-select action buttons
* Implement multi-read/unread
* Implement multi-favorites (stars)
* Implement multi-delete
TODO: menu items when there are selected items
* 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.
