Fixes Issue 300

Fixes Issue 772
Fixes Issue 852

Issue 300: Handle incoming IMAP tokens in a case-insensitive manner

Issue 772: Add Yahoo workaround

Issue 852: Add configurable IMAP IDLE refresh frequency in Incoming
server settings

Also:
Add configuration option for whether push system executes a poll on
each connect.

Move the configuration for the maximum number of push folders to the
Incoming server Settings.