A worker thread can now have its own log target which will be used directly
by the log functions instead of buffering log output in the main thread; the
GUI thread in the thread sample shows how it works.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61422 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775