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