[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()

src/Mod/TechDraw/Gui/DlgPrefsTechDraw2Imp.cpp

@@ -36,6 +36,8 @@ DlgPrefsTechDraw2Imp::DlgPrefsTechDraw2Imp( QWidget* parent )
     this->setupUi(this);
 
     this->pdsbTemplateMark->setUnit(Base::Unit::Length);
+    this->pdsbTemplateMark->setMinimum(0);
+
     connect(this->cbViewScaleType, SIGNAL(currentIndexChanged(int)),
         this, SLOT(onScaleTypeChanged(int)));
 }

src/Mod/TechDraw/Gui/DlgPrefsTechDraw3Imp.cpp

@@ -43,8 +43,11 @@ DlgPrefsTechDraw3Imp::DlgPrefsTechDraw3Imp( QWidget* parent )
 {
     this->setupUi(this);
     plsb_FontSize->setUnit(Base::Unit::Length);
+    plsb_FontSize->setMinimum(0);
     plsb_ArrowSize->setUnit(Base::Unit::Length);
+    plsb_ArrowSize->setMinimum(0);
     pdsbBalloonKink->setUnit(Base::Unit::Length);
+    pdsbBalloonKink->setMinimum(0);
 }
 
 DlgPrefsTechDraw3Imp::~DlgPrefsTechDraw3Imp()