Add missing parent for messagebox and other popups

Unparented popups can easily get lost in backround, but they still block top level event loop when run with ::exec() thus preventing interactions with main window.  This mainly happens on wayland. Setting the parent ensures they are always kept on top and reasonably positioned.

src/Mod/Fem/Gui/ViewProviderFemPostObject.cpp

@@ -964,7 +964,7 @@ bool ViewProviderFemPostObject::setEdit(int ModNum)
             postDlg = nullptr;  // another pad left open its task panel
         }
         if (dlg && !postDlg) {
-            QMessageBox msgBox;
+            QMessageBox msgBox(Gui::getMainWindow());
             msgBox.setText(QObject::tr("A dialog is already open in the task panel"));
             msgBox.setInformativeText(QObject::tr("Do you want to close this dialog?"));
             msgBox.setStandardButtons(QMessageBox::Yes | QMessageBox::No);