Changes to make it possible to write multi-threaded programs using

Changes to make it possible to write multi-threaded programs using
Tkinter.  This adds a separate lock -- read the comments.  (This was
also needed for Mark Hammond's attempts to make PythonWin
Tkinter-friendly.)

The changes have affected the EventHook slightly, too; and I've done
some more cleanup of the code that deals with the different versions
of Tcl_CreateFileHandler().