Add setCombinedInbox binding.

setCombinedInbox sets the MessageList model to show messages from
the inbox of all enabled accounts, there's also a option to show
only unread messages in the model.
Detect inbox folder using the Standard folders flags.

Current inbox detection does not work for pop accounts were
inbox is called "Inbox".
Merge pull request #47 from VDVsx/master

 Add setCombinedInbox binding