Gui: Allow users to add groups to active objects

As the title says, if right now there is Arch type active (like Level,
Building, etc. etc.), then it's not possible to assign Group to it
automatically (it's being created on root level of the document).

So this patch basically takes an active object and tries to insert it.

src/Gui/CommandStructure.cpp

@@ -32,6 +32,7 @@
 #include "ActiveObjectList.h"
 #include "Application.h"
 #include "Document.h"
+#include "MDIView.h"
 #include "ViewProviderDocumentObject.h"
 #include "Selection.h"
 
@@ -121,9 +122,27 @@ void StdCmdGroup::activated(int iMsg)
     std::string GroupName;
     GroupName = getUniqueObjectName("Group");
     QString label = QApplication::translate("Std_Group", "Group");
-    doCommand(Doc,"App.activeDocument().Tip = App.activeDocument().addObject('App::DocumentObjectGroup','%s')",GroupName.c_str());
-    doCommand(Doc,"App.activeDocument().%s.Label = '%s'", GroupName.c_str(),
-              label.toUtf8().data());
+
+    // create a group
+    doCommand(Doc,"group = App.activeDocument().addObject('App::DocumentObjectGroup','%s')",GroupName.c_str());
+    doCommand(Doc,"group.Label = '%s'", label.toUtf8().data());
+    doCommand(Doc,"App.activeDocument().Tip = group");
+
+    // try to add the group to any active object that supports grouping (has GroupExtension)
+    if (auto* activeDoc = Gui::Application::Instance->activeDocument()) {
+        if (auto* activeView = activeDoc->getActiveView()) {
+            // find the first active object with GroupExtension
+            if (auto* activeObj = activeView->getActiveObjectWithExtension(
+                    App::GroupExtension::getExtensionClassTypeId())) {
+                doCommand(Doc,
+                          "active_obj = App.activeDocument().getObject('%s')\n"
+                          "if active_obj and active_obj.allowObject(group):\n"
+                          "    active_obj.Group += [group]",
+                          activeObj->getNameInDocument());
+            }
+        }
+    } // if we have no active object, group will be added to root doc
+
     commitCommand();
 
     Gui::Document* gui = Application::Instance->activeDocument();