(Backport filter fix from master)

A biggish refactoring of the filter functionality:

1) Do not take the items from the cache only, as they might be incomplete (especially in online imap case, but not only).
This should fix the body loss bug on spam filtering.(Bug 287752 and similar ones + countless of reports on user list)
Might fix 293768.

2) Download only what is really needed for the filter and add safety checks that
the mail is not modified if the requested part is missing. So far the distinction
was only betwee the body and rest. This should speed up filtering.
Also related to 287752, but should fix also 292283, 288109.

3) Rename qDebug->kDebug + fix some includes

4) The code should also fix the duplicate mails appearing after filtering (some of the duplicates
with empty content), eg. 275233

All bug reporters: in case you still see the bug, reopen the corresponding one.
For the reports that were CC'd, but not closed, please try to reproduce and tell
the result.

So far the bug is fixed on master, but might be backported later to 4.9.

BUG: 287752
BUG: 292283
BUG: 288109
BUG: 286043
BUG: 295484
BUG: 302337
BUG: 295000
BUG: 295684
CCBUG: 293768
CCBUG: 275233
CCBUG: 293918
CCBUG: 284310
CCBUG: 286364
CCBUG: 283682
(cherry picked from commit d64d61b4)

FIXED-IN: 4.9