avoid memory leak in slave jobs
Summary:
Making ~SlaveInterface virtual, the connection created in Slave constructor is now deleted. (thanks @dfaure).
Changed to new connect syntax.
BUG: 396651
Test Plan:
The leaks of the bug report are gone.
Reviewers: dfaure, Frameworks, ngraham
Reviewed By: dfaure
Subscribers: anthonyfieroni, apol, kde-frameworks-devel
Tags: Frameworks
Differential Revision: https://phabricator.kde.org/D14253