Gui: Prevent unhandled exception adding properties

This commit prevents unhandled exceptions to be thrown in the Add
Property dialog.  When the user provides a valid name, for example 'Abc'
and then an invalid name, for example 'Abc^', an exception is thrown
that is caught by Qt's notify which may lead to a crash.  This is caused
by the fact that the editor is still bound to the old property that has
been removed.  The main idea was to preserve values that were provided
by the user as long as possible, also when a property name changes.
This design goal tried to preserve the user provided value even if the
property name is not valid.  Although the underlying property item is
explicitly unbound in that situation, the editor is not.

The solution is to also explicitly unbind the editor if there is an
editor that can have bindings.  Unbinding an editor does not
automatically hide the f(x) symbol in the editor (leading to errors on
unbound editors), so this commit also adds a bit of logic to hide
the f(x) symbol when an editor is unbound.

src/Gui/Dialogs/DlgAddProperty.cpp

@@ -804,6 +804,9 @@ bool DlgAddProperty::clearBoundProperty()
 
     if (App::Property* prop = propertyItem->getFirstProperty()) {
         propertyItem->unbind();
+        if (auto* eb = dynamic_cast<ExpressionBinding*>(editor.get())) {
+            eb->unbind();
+        }
         propertyItem->removeProperty(prop);
         container->removeDynamicProperty(prop->getName());
         closeTransaction(TransactionOption::Abort);

src/Gui/ExpressionBinding.cpp

@@ -334,6 +334,12 @@ QPixmap ExpressionWidget::getIcon(const char* name, const QSize& size) const
     return icon;
 }
 
+void ExpressionWidget::unbind()
+{
+    iconLabel->hide();
+    ExpressionBinding::unbind();
+}
+
 void ExpressionWidget::makeLabel(QLineEdit* le)
 {
     defaultPalette = le->palette();

src/Gui/ExpressionBinding.h

@@ -49,7 +49,7 @@ public:
     virtual void bind(const App::ObjectIdentifier& _path);
     virtual void bind(const App::Property& prop);
     bool isBound() const;
-    void unbind();
+    virtual void unbind();
     virtual bool apply(const std::string& propName);
     virtual bool apply();
     bool hasExpression() const;
@@ -99,6 +99,7 @@ class GuiExport ExpressionWidget: public ExpressionBinding
 public:
     ExpressionWidget();
     QPixmap getIcon(const char* name, const QSize& size) const;
+    void unbind() override;
 
 protected:
     void makeLabel(QLineEdit* parent);