Issue 4: Implements Push Mail for IMAP accounts using IMAP IDLE

This commit contains the entirety of the changes performed in the
issue4-1.X branch from revision 718 through revision 851.  Because the
issue4-1.X branch was up-to-date with trunk revision 847 at revision
849, the source of this commit was not an "svn merge".  Instead, it is
merely a copy of all changed files from the issue4-1.X branch to my
trunk working copy and a straight commit.

Also:
Issue 551
Issue 628
Issue 650
Issue 654
Issue 656
Issue 682
Issue 696