travis: adapt the CI builds for changed location of the cmake backport
tests: make the LITERAL testing more robust

It was a pretty stupid idea to rely on fragile input -- the string length
depends on the Qt version, locale settings etc etc. My bad for not catching this
earlier.

BUG: 322029
REVIEW: 111489
Fix typos in some comments

REVIEW: 111311
Fix link failure on Windows (MinGW)

Patch by "OlegYch" <olegych@tut.by> (no real name available).

BUG: 321802
rfc1951: Fix deleting buffer in destructor

Array _buffer is allocated by new char[chunkSize], so must be freed by delete[] operator instead delete.

REVIEW: 111490
Gui: SettingsDialog: For SMTP and SSMTP methods enable Save Outgoing Mail checkbox

When user click on IMAP SENDMAIL method, Trojita automatically check
Save Outgoing Mail checkbox and disable it. When user again select SMTP
method Trojita should re-enable this checkbox and allow user to uncheck
this checkbox.

REVIEW: 111495