Gui: Make VarSet dialog modal

Also tracks its own transactions to prevent interfering with other
transactions.

src/Gui/ViewProviderVarSet.cpp

@@ -46,6 +46,10 @@ bool ViewProviderVarSet::doubleClicked()
         dialog = std::make_unique<DlgAddPropertyVarSet>(getMainWindow(), this);
     }
 
+    // Do not use exec() here because it blocks and prevents command Std_VarSet
+    // to commit the autotransaction.  This in turn prevents the dialog to
+    // handle transactions well.
+    dialog->setWindowModality(Qt::ApplicationModal);
     dialog->show();
     dialog->raise();
     dialog->activateWindow();