Functionality enhancement: allow other threads to use Tk commands

while one thread is blocked in mainloop().  Also, handle signals (not
just interrupts) as soon as they happen.

Cleanup: remove support for Tcl/Tk versions 7.4/4.0.  (I've confirmed
that it works for 7.5/4.1 and 7.6/4.2, as well as 8.0b2.)

Coding style change: instead of ``func (args)'', write ``func(args)''
everywhere.

Minor functionality change: use PyArg_ParseTuple everywhere.  This
should only affect the errors reported for bad argument lists; in
particular, deletefilehandler() is much clearer about what's going
on.

(XXX Still to do: Mac and Win ports to 8.0b2.)