Allow wxThread::Wait() and Delete() to block, even under wxMSW.

Add "wait mode" parameter to these methods which can be used to make them
block even under wxMSW where they currently dispatch messages when called
which can be totally unexpected.

Do keep the old behaviour for compatibility however, although it will change i
3.2.

Closes #12998.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67185 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775