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/Gui/TaskView/TaskDialog.cpp

@@ -30,6 +30,7 @@
 #include <App/Document.h>
 #include <Gui/Application.h>
 #include <Gui/Document.h>
+#include <Gui/MainWindow.h>
 #include <Gui/View3DInventor.h>
 #include <Gui/ViewProviderDocumentObject.h>
 
@@ -96,7 +97,7 @@ const std::vector<QWidget*> &TaskDialog::getDialogContent() const
 
 bool TaskDialog::canClose() const
 {
-    QMessageBox msgBox;
+    QMessageBox msgBox(Gui::getMainWindow());
     msgBox.setText(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);