App/Gui: introduce temporary document

Add new argument to Application::newDocument() to create a temporary
    document. Also exposed to Python API App.newDocument() with a named
    argument 'temp'.

    The temporary document is marked with status bit 'TempDoc'. The user
    will not be prompt for saving when closing. The undo/redo is disabled.
    The AutoSaver skips it. And the tree view will not show it.
    PropertyXLink allows linking to/from object within a temporary document
    without saving.

src/App/Document.cpp

@@ -2894,6 +2894,12 @@ const char* Document::getProgramVersion() const
     return d->programVersion.c_str();
 }
 
+const char* Document::getFileName() const
+{
+    return testStatus(TempDoc) ? TransientDir.getValue()
+                               : FileName.getValue();
+}
+
 /// Remove all modifications. After this call The document becomes valid again.
 void Document::purgeTouched()
 {
@@ -4241,7 +4247,7 @@ void Document::breakDependency(DocumentObject* pcObject, bool clear)
 }
 
 std::vector<DocumentObject*> Document::copyObject(
-    const std::vector<DocumentObject*> &objs, bool recursive)
+    const std::vector<DocumentObject*> &objs, bool recursive, bool returnAll)
 {
     std::vector<DocumentObject*> deps;
     if(!recursive)
@@ -4249,10 +4255,11 @@ std::vector<DocumentObject*> Document::copyObject(
     else
         deps = getDependencyList(objs,DepNoXLinked|DepSort);
 
-    if(!isSaved() && PropertyXLink::hasXLink(deps))
+    if (!testStatus(TempDoc) && !isSaved() && PropertyXLink::hasXLink(deps)) {
         throw Base::RuntimeError(
                 "Document must be saved at least once before link to external objects");
-        
+    }
+
     MergeDocuments md(this);
     // if not copying recursively then suppress possible warnings
     md.setVerbose(recursive);
@@ -4291,7 +4298,7 @@ std::vector<DocumentObject*> Document::copyObject(
         imported = md.importObjects(istr);
     }
 
-    if(imported.size()!=deps.size())
+    if (returnAll || imported.size()!=deps.size())
         return imported;
 
     std::unordered_map<App::DocumentObject*,size_t> indices;