mac: Wait a bit before activating app at startup.

This is obnoxious and wrong, but the patch that activates the Dock before
activating the app fixes the _menu_ not responding on Catalina, but the
first window created by the app won't have keyboard focus without a small
delay inserted.

This obviously needs a better solution, but it gets it limping along correctly
for now.

src/video/cocoa/SDL_cocoaevents.m

@@ -244,6 +244,7 @@ static void Cocoa_DispatchEvent(NSEvent *theEvent)
             [i activateWithOptions:NSApplicationActivateIgnoringOtherApps];
             break;
         }
+        SDL_Delay(300);  /* !!! FIXME: this isn't right. */
         [NSApp activateIgnoringOtherApps:YES];
     }