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

src/Gui/QuantitySpinBox.h

@@ -153,6 +153,10 @@ Q_SIGNALS:
      *  like: minimum, maximum and/or the right Unit (if specified).
      */
     void valueChanged(double);
+    /** Gets emitted if formula dialog is about to be opened (true)
+     *  or finished (false).
+     */
+    void showFormulaDialog(bool);
 
 private:
     QScopedPointer<QuantitySpinBoxPrivate> d_ptr;

src/Mod/Sketcher/Gui/InsertDatum.ui

@@ -48,6 +48,19 @@
      </item>
     </layout>
    </item>
+   <item>
+    <spacer name="verticalSpacer">
+     <property name="orientation">
+      <enum>Qt::Vertical</enum>
+     </property>
+     <property name="sizeHint" stdset="0">
+      <size>
+       <width>20</width>
+       <height>40</height>
+      </size>
+     </property>
+    </spacer>
+   </item>
    <item>
     <widget class="QDialogButtonBox" name="buttonBox">
      <property name="orientation">
@@ -65,6 +78,9 @@
    <class>Gui::PrefQuantitySpinBox</class>
    <extends>QWidget</extends>
    <header>Gui/PrefWidgets.h</header>
+   <slots>
+    <signal>showFormulaDialog(bool)</signal>
+   </slots>
   </customwidget>
  </customwidgets>
  <resources/>
@@ -101,5 +117,21 @@
     </hint>
    </hints>
   </connection>
+  <connection>
+   <sender>labelEdit</sender>
+   <signal>showFormulaDialog(bool)</signal>
+   <receiver>buttonBox</receiver>
+   <slot>setHidden(bool)</slot>
+   <hints>
+    <hint type="sourcelabel">
+     <x>212</x>
+     <y>26</y>
+    </hint>
+    <hint type="destinationlabel">
+     <x>171</x>
+     <y>101</y>
+    </hint>
+   </hints>
+  </connection>
  </connections>
 </ui>