do not add debugger button unless it is meant to be visible

Summary:
QDBBox explicitly calls show() and hide() all over the place ultimately
resulting in the button being visible when in fact we do not want it to
be.

Test Plan:
- without patch I always get a debug button despite showExternalDebuggers returning false
- with patch I don't

Reviewers: broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D5080