Don't block in wxEventLoopManual::Dispatch() if loop was exited.

Don't block in wxEventLoopManual::Dispatch() if loop was exited.

If Exit() was called from a handler for one of the pending events we could
reenter Dispatch() and block there indefinitely if no other events were coming
and this was exactly what happened in wxFileSystemWatcher unit test,
preventing it from ever running to completion under Unix.

Fix this by checking m_shouldExit after executing the pending handlers and
before calling Dispatch().

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71304 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775