App: Add temporaryDocument to DocumentObject

This is used for detached DocumentObjects before they're attached to the document to support functionality that depends on a valid document

src/App/Document.cpp

@@ -3388,7 +3388,7 @@ std::vector<DocumentObject *> Document::addObjects(const char* sType, const std:
 
 void Document::addObject(DocumentObject* pcObject, const char* pObjectName)
 {
-    if (pcObject->getDocument()) {
+    if (pcObject->isAttachedToDocument()) {
         throw Base::RuntimeError("Document object is already added to a document");
     }
 

src/App/DocumentObject.cpp

@@ -687,15 +687,23 @@ void DocumentObject::onLostLinkToObject(DocumentObject*)
 
 App::Document *DocumentObject::getDocument() const
 {
+    if (!_pDoc)
+        return _pTempDoc;
     return _pDoc;
 }
 
 void DocumentObject::setDocument(App::Document* doc)
 {
+    _pTempDoc = nullptr;
     _pDoc=doc;
     onSettingDocument();
 }
 
+void DocumentObject::setTemporaryDocument(App::Document* doc)
+{
+    _pTempDoc = doc;
+}
+
 bool DocumentObject::removeDynamicProperty(const char* name)
 {
     if (!_pDoc || testStatus(ObjectStatus::Destroy)) 

src/App/DocumentObject.h

@@ -151,6 +151,8 @@ class AppExport DocumentObject: public App::TransactionalObject
     const char* detachFromDocument() override;
     /// gets the document in which this Object is handled
     App::Document *getDocument() const;
+    /// sets the temporary document for detached objects which will be used as a fallback
+    void setTemporaryDocument(App::Document* doc);
 
     /** Set the property touched -> changed, cause recomputation in Update()
      */
@@ -671,6 +673,9 @@ class AppExport DocumentObject: public App::TransactionalObject
     /// pointer to the document this object belongs to
     App::Document* _pDoc{nullptr};
 
+    /// pointer to the temporary document for detached objects
+    App::Document* _pTempDoc{nullptr};
+
     /// Old label; used for renaming expressions
     std::string oldLabel;