Add initial prototype UI for selecting a per-identity sent folder.
Open the ChooseFolder activity to pick a per-identity sent folder, and
update the TextView to reflect the chosen folder.
Save and load the per-identity sent folder to/from the preferences.
Fix wrong comment.
When requesting a message to be sent, keep a record of the sent folder
to be used in the temporary X-K9mail-Sent-Folder header, and pick this
up in the MessagingController when actually sending it.
Remove the added header before sending, to avoid leaking it in the sent
message.

Note: AFAICS, the header remains in the outbox message (which is later
moved to the sent folder). This is good I think, because if the message fails to
send, then the correct folder will still be used on a later retry.
I don't think there's a problem with this field remaining in the sent folder.
Preserve the sent folder to use across the message being saved as a draft.

I wasn't sure here if I should perhaps store this in the X-K9mail-Identity
header, as it seems like that is the place for storing 'Drafty' things.
However, it seemed a bit pointless as we are storing it already in another
header.
Some coding style fixes.