Fix 8 memory-leaks by cleaning up created Tcl commands
automatically.

I attach a patch against Tkinter 47021.

=== Long explanation ===

I was bitten by a memory leak in Tkinter - 25MB per day
on a long-running process.  A net search found a couple
unrelated Tkinter leaks, and gave me some clues. 
Investigation using the tracing feature in tkleak.py
(see link) found the bug.

I searched for more similar leaks, and fixed them too.

The reasoning for patch #1121234 gives the reason for
the changes to _register() and deletecommand().

See http://www.uk.debian.org/~graham/python/tkleak.py
for my leak tracing and test script.