Fix German translation of SMTP error message: Closes bgo#723221
Fixes IMAP error causing all remote folders to be considered deleted
Better ClientSession handling in Imap.Account

Simplifies handling connection drops, which occur more frequently
when AUTHORIZED rather than SELECTED/EXAMINED w/ IDLE.
Fix regression where folders were not regularly updated w/ STATUS info

Now that Imap.Folders are being preserved, need to update their
FolderProperties w/ STATUS results whenever listed.
Prevent reconnect loop from occurring

I noticed recently that when composing a draft message, if the
connection to the Drafts folder was dropped by the server, Geary
would enter a fast reconnect loop.  The problem was that, even if
Imap.Folder signals "disconnect", it's close_async() must be called.
This adds that logic to the background reestablishment code.
Fix unstarred icon's interior from selection color: Closes bgo#714426

This deals with both the star and unread icon, so both are good now.
Need to clean up the rest of the conversation list's selection probs,
bgo#723265.  Also would like to reduce the intensity of the icons,
bgo#720771.
Add option to save sent mail

This adds the ability for Geary to push sent mail up to the account's
Sent Mail folder (if available).  There's an accompanying account option
that defaults to on (meaning: push sent mail).

The current implementation will leave messages in the Outbox (though
they won't be sent again) if they fail to be pushed to Sent Mail.  This
isn't the best solution, but it at least means you have a way of seeing
the problem and hopefully copying the data elsewhere manually if you
need to save it.

Note that Geary might not always recognize an account's Sent Mail
folder.  This is the case for any "Other" accounts that don't support
the "special use" or "xlist" IMAP extensions.  In this case, Geary will
either throw an error and leave messages in the Outbox, or erase the
message from the Outbox when it's sent, depending on the value of the
account's save sent mail option.  Better support for detecting the Sent
Mail folder in every case is coming soon.

Closes: bgo #713263
Merge branch 'master' into iterate
Fix last usage of SingleItem from master