Set m_isShowingModal as true before showing dialog in wxUniv

This allows ModalDialogsTestCase InitDialog unit test to pass.

Closes https://github.com/wxWidgets/wxWidgets/pull/2481

src/univ/dialog.cpp

@@ -181,10 +181,8 @@ int wxDialog::ShowModal()
     {
         m_parent = parent;
     }
-
-    Show(true);
-
     m_isShowingModal = true;
+    Show(true);
 
     wxASSERT_MSG( !m_windowDisabler, wxT("disabling windows twice?") );