replace custom backtracing in SlaveBase with KCrash

Summary:
as discussed on mailing list.

KCrash originally was automatically initialized, but now needs to be
explicitly linked to work.
additionally the custom backtrace() in the SlaveBase is only marginally
useful and largely replaced by improvements to KCrash (such as core pattern
support on linux to allow raising crashes to a global core collector such
as systemd's coredumpd).

If someone wants to resurrect the custom backtrace() feature it needs to
be moved to KCrash and actually made signal-safe (explicitly load libgcc;
see backtrace manpage).

CHANGELOG: KIO slave crashes are now handled by KCrash instead of subpar custom code

Test Plan: - q_assert(false) in a slave now brings up drkonqi

Reviewers: dfaure

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D15573