Don't fetch deleted messages from IMAP folder

Fetching messages from an IMAP folder within a specific date range returns
messages marked as deleted, also. Unlike every other getMessages() method,
which filter out deleted messages.
This leaves server deleted messages untouched in the mail conversation
view, even when the folder is synced manually.

This patch adds "NOT DELETED" to the IMAP search command, when searching
for mails in a given date range.

Change-Id: I49e4adfcab9a1de1d1baa4700462a82df651f3cb