Don't allow accelerators intercept F4 in the MSW comboboxes

This key is used to open the dropdown and should still work even if
there is an accelerator using it too.

src/msw/choice.cpp

@@ -139,6 +139,10 @@ bool wxChoice::MSWShouldPreProcessMessage(WXMSG *msg)
                     return false;
                 }
                 break;
+
+            case VK_F4:
+                // This key can always be used to show the dropdown.
+                return false;
         }
     }