Require _tkinter -- don't attempt to import tkinter when _tkinter does

Require _tkinter -- don't attempt to import tkinter when _tkinter does
not exist.  All 8 uses of tkinter are replaced with _tkinter.  Still
create a variable tkinter though, because that is used by other
modules importing Tkinter (e.g. tkinter.createfilehandler()).

Also added a comment to the 'import _tkinter' line saying that if this
fails, Python is not configured correctly.