Reduce size of general debugger settings.

Part of a series of commits reducing the (vertical) size of the
preferences dialog.

Task-number: QTCREATORBUG-3172
Change-Id: I649ef392a8fc0fb068cc2f683d58ef324981a1b5
Reviewed-on: http://codereview.qt-project.org/5232


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: hjk <qthjk@ovi.com>