[TD] deny input of negative lengths in pref dialogs

see https://forum.freecadweb.org/viewtopic.php?f=35&t=44374&p=379207#p379207

src/Mod/TechDraw/Gui/DlgPrefsTechDraw1Imp.cpp

@@ -35,6 +35,7 @@ DlgPrefsTechDraw1Imp::DlgPrefsTechDraw1Imp( QWidget* parent )
 {
     this->setupUi(this);
     plsb_LabelSize->setUnit(Base::Unit::Length);
+    plsb_LabelSize->setMinimum(0);
 }
 
 DlgPrefsTechDraw1Imp::~DlgPrefsTechDraw1Imp()