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BUG: 333212
REVIEW: 117514
Get rid of the duplicated FLagsOperation

The QML is rather hostile towards working with enums which come from the C++
code *and* are not already wrapped in a QObject-deriving class. Instead of
duplicating the existing FlagsOperation enum partially, it seems that it is
cleaner to give up on the API design principles which ask for using enums
instead of plain bools for readability, and use a simple bool in this case.
Merge branch 'ubuntu-delete-message'
No need for "explicit" in this ctor anymore
IMAP: relax quoted-string parsing to allow for Groupwise's escaped parentheses

As per the bugreport, Groupwise does not conform to RFC3501 and sends
parentheses ("(" and ")") as escaped characters within the quoted strings. The
RFC 3501 is pretty clear in only allowing escaped quotes and backslashes.

This change let's Trojita process these broken responses. It is a tough call to
tell what to do, but interpretting these by ignoring the backslashes is probably
the lesser of the two evils.

BUG: 334456
REVIEW: 118033
IMAP: Accept Exchange's botched body-fld-dsp

According to a report by Martin Steigerwald <ms@teamix.de>, Exchange sends a
body-fld-dsp which contains complete crap.

CCBUG: 334056
REVIEW: 118035