Focus the contents of the generic preferences dialog initially

This is more useful than leaving the focus on the "OK" button, which can
already be easily activated from keyboard.

Closes #19277.

src/generic/preferencesg.cpp

@@ -58,6 +58,8 @@ public:
                    wxSizerFlags().Expand().DoubleBorder(wxLEFT|wxRIGHT|wxBOTTOM));
 #endif
         SetSizer(sizer);
+
+        m_notebook->SetFocus();
     }
 
     void AddPage(wxPreferencesPage *page)