[TD] Rewrite balloon placing method

When clicking on balloon command, cursor is changing to balloon cursor.
It means user is ready to place a balloon (anywhere on the page).
Balloon object is not created until user click somewhere.

Balloon command can be aborted pressing escape or when page view lost
focus (in other words, when user clic somewhere else than the page).

src/Mod/TechDraw/App/DrawPage.cpp

@@ -104,6 +104,7 @@ DrawPage::DrawPage(void)
     Scale.setConstraints(&scaleRange);
     double defScale = hGrp->GetFloat("DefaultScale",1.0);
     Scale.setValue(defScale);
+    balloonPlacing = false;
 }
 
 DrawPage::~DrawPage()