[TechDraw] let arrow size of a dimension change due property task panel

src/Mod/TechDraw/App/Preferences.cpp

@@ -42,6 +42,7 @@
 using namespace TechDraw;
 
 const double Preferences::DefaultFontSizeInMM = 5.0;
+const double Preferences::DefaultArrowSize = 3.5;
 
 //! Returns the TechDraw preference group
 Base::Reference<ParameterGrp> Preferences::getPreferenceGroup(const char* Name)
@@ -70,6 +71,11 @@ double Preferences::dimFontSizeMM()
     return getPreferenceGroup("Dimensions")->GetFloat("FontSize", DefaultFontSizeInMM);
 }
 
+double Preferences::dimArrowSize()
+{
+    return getPreferenceGroup("Dimensions")->GetFloat("ArrowSize", DefaultArrowSize);
+}
+
 App::Color Preferences::normalColor()
 {
     App::Color fcColor;