Fix Balloon Next Index Persistance

- next sequential number for balloon on Page was
  not being preserved across save/restore

src/Mod/TechDraw/App/DrawPage.cpp

@@ -101,6 +101,9 @@ DrawPage::DrawPage(void)
     }
 
     ADD_PROPERTY_TYPE(Scale, (1.0), group, (App::PropertyType)(App::Prop_None), "Scale factor for this Page");
+    ADD_PROPERTY_TYPE(NextBalloonIndex, (1), group, (App::PropertyType)(App::Prop_None),
+                     "Auto-numbering for Balloons");
+
     Scale.setConstraints(&scaleRange);
     double defScale = hGrp->GetFloat("DefaultScale",1.0);
     Scale.setValue(defScale);
@@ -406,6 +409,14 @@ void DrawPage::unsetupObject()
     Template.setValue(nullptr);
 }
 
+int DrawPage::getNextBalloonIndex(void)
+{
+    int result = NextBalloonIndex.getValue();
+    int newValue = result + 1;
+    NextBalloonIndex.setValue(newValue);
+    return result;
+}
+
 void DrawPage::Restore(Base::XMLReader &reader)
 {
     reader.readElement("Properties");

src/Mod/TechDraw/App/DrawPage.h

@@ -49,6 +49,8 @@ public:
 
     App::PropertyFloatConstraint Scale;
     App::PropertyEnumeration ProjectionType; // First or Third Angle
+    
+    App::PropertyInteger  NextBalloonIndex;
 
     /** @name methods override Feature */
     //@{
@@ -91,7 +93,9 @@ public:
     void requestPaint(void);
     std::vector<App::DocumentObject*> getAllViews(void) ;
     bool balloonPlacing;
-    DrawViewPart *balloonParent;
+    DrawViewPart *balloonParent;    //could be many balloons on page? 
+    
+    int getNextBalloonIndex(void);
 
 protected:
     void onBeforeChange(const App::Property* prop);