App: Fix output string to XML

Not all unicode characters are allowed as XML output. When writing disallowed characters the SAX parser throws an exception
when loading a project file that results into a broken document and thus to a possible loss of data.

This PR replaces all disallowed characters with an underscore and prints a warning.

This fixes https://github.com/FreeCAD/FreeCAD/issues/22123
Note: It does not fix an already corrupted project file.

src/App/PropertyStandard.cpp

@@ -1509,6 +1509,14 @@ void PropertyString::setPyObject(PyObject* value)
 
 void PropertyString::Save(Base::Writer& writer) const
 {
+    auto verifyXMLString = [this](std::string& input) {
+        const std::string output = this->validateXMLString(input);
+        if (output != input) {
+            Base::Console().warning("XML output: Validate invalid string:\n'%s'\n'%s'\n",
+                                    input, output);
+        }
+        return output;
+    };
     std::string val;
     auto obj = freecad_cast<DocumentObject*>(getContainer());
     writer.Stream() << writer.ind() << "<String ";
@@ -1520,11 +1528,13 @@ void PropertyString::Save(Base::Writer& writer) const
         else if (_cValue == obj->getNameInDocument()) {
             writer.Stream() << "restore=\"0\" ";
             val = encodeAttribute(obj->getExportName());
+            val = verifyXMLString(val);
             exported = true;
         }
     }
     if (!exported) {
         val = encodeAttribute(_cValue);
+        val = verifyXMLString(val);
     }
     writer.Stream() << "value=\"" << val << "\"/>" << std::endl;
 }