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/Assembly/Gui/ViewProviderAssembly.cpp

@@ -55,6 +55,7 @@
 #include <Gui/CommandT.h>
 #include <Gui/Control.h>
 #include <Gui/MDIView.h>
+#include <Gui/MainWindow.h>
 #include <Gui/SoFCCSysDragger.h>
 #include <Gui/View3DInventor.h>
 #include <Gui/View3DInventorViewer.h>
@@ -191,7 +192,7 @@ bool ViewProviderAssembly::canDragObjectToTarget(App::DocumentObject* obj,
         if (obj == obj1 || obj == obj2 || obj == part1 || obj == part2 || obj == obj3) {
             if (!prompted) {
                 prompted = true;
-                QMessageBox msgBox;
+                QMessageBox msgBox(Gui::getMainWindow());
                 msgBox.setText(tr("The object is associated to one or more joints."));
                 msgBox.setInformativeText(
                     tr("Do you want to move the object and delete associated joints?"));