Fix "Enter" in wxComboBox with wxTE_PROCESS_ENTER in wxMSW

Activate the default button of the dialog containing the wxComboBox if
wxEVT_TEXT_ENTER handler didn't process the even, just as it was already
done for wxTextCtrl.

See #18273.

src/msw/combobox.cpp

@@ -243,6 +243,9 @@ bool wxComboBox::MSWProcessEditSpecialKey(WXWPARAM vkey)
                     // beep if it gets it
                     return true;
                 }
+
+                if ( ClickDefaultButtonIfPossible() )
+                    return true;
             }
             break;