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/PartDesign/Gui/TaskDatumParameters.cpp

@@ -97,7 +97,7 @@ bool TaskDlgDatumParameters::accept() {
 
     //see if we are able to assign a mode
     if (parameter->getActiveMapMode() == mmDeactivated) {
-        QMessageBox msg;
+        QMessageBox msg(Gui::getMainWindow());
         msg.setWindowTitle(tr("Incompatible reference set"));
         msg.setText(tr("There is no attachment mode that fits the current set"
         " of references. If you choose to continue, the feature will remain where"