Improve usability of expression dialog:

+ add signal showFormulaDialog to QuantitySpinBox in order to indicate whether the formula dialog opens or closes
+ when formula dialog opens then hide the button box in the parent dialog, show it again when the formula dialog closes

src/Gui/QuantitySpinBox.cpp

@@ -587,6 +587,8 @@ void QuantitySpinBox::openFormulaDialog()
     QPoint pos = mapToGlobal(QPoint(0,0));
     box->move(pos-box->expressionPosition());
     box->setExpressionInputSize(width(), height());
+
+    Q_EMIT showFormulaDialog(true);
 }
 
 void QuantitySpinBox::finishFormulaDialog()
@@ -603,6 +605,8 @@ void QuantitySpinBox::finishFormulaDialog()
         setExpression(boost::shared_ptr<Expression>());
 
     box->deleteLater();
+
+    Q_EMIT showFormulaDialog(false);
 }
 
 Base::Unit QuantitySpinBox::unit() const