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