Make sure that subwindows won’t leak out of their parent window 
even if they have coordinates that would provoke it.  

git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10475 ea41ed52-d2ee-0310-a9c1-e6b18d33e121
Extend advanced.dox multithreading to add additional caveats
about the use of show() and hide() from child threads, based
on issues reported in fltk.general with tooltip windows
causing issues.

git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10476 ea41ed52-d2ee-0310-a9c1-e6b18d33e121