Made wxLogXXX() functions thread-safe.

Made wxLogXXX() functions thread-safe.

They can now be called from any thread and will buffer the messages until the
current log target is flushed from the main thread. This makes earlier code to
do the same thing specifically for wxLogWindow unnecessary and also allows to
use wxLogMessage() in the thread sample instead of using manual logging there.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61416 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775