Fix OS X startup to not hang if the application doesn't get focus.

If the application launches while the parent process doesn't have an active
window, [NSApp run] won't terminate immediately, as was assumed here. Instead,
it blocks until some input arrives, e.g. clicking the Dock icon.

Work around this by adding a dummy event to the queue.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75302 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775