Fix Bug 374458 by properly blocking signals on debugger combo box

Summary:
Fix https://bugs.kde.org/show_bug.cgi?id=374458

In the launch configuration dialog, when changing config page and updating the debugger combo box, signals are not properly
blocked, thus triggering incorrect `setData` call on the model. Bug 374458 happens only when GDB is choosen as debugger because GDB
happens to be the second item in the combo box.

Test Plan: Maunally tested, no more crash.

Reviewers: apol, #kdevelop, flherne

Reviewed By: #kdevelop, flherne

Subscribers: flherne, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D4555