Revise pdbtrack functionality to incorporate advances in python-mode.el.

Revise pdbtrack functionality to incorporate advances in python-mode.el.

(I'm doing this python.el checkin with some byte-compiler warnings.  These
warnings existed before Nick Roberts or I applied any of the pdbtrack
changes, and look very clearly like preexisting, incomplete adoption of
code from python-mode.el.  I'm going to next look at settling those
warnings, though I don't have time for a major reconciliation of the two
python-mode implementations.)

(python-pdbtrack-toggle-stack-tracking): Clarify docstring.
(python-pdbtrack-minor-mode-string): A sign indicating that pdb
tracking is happening.
(python-pdbtrack-stack-entry-regexp): Better recognize stack traces.
(python-pdbtrack-input-prompt): Better recognize PDB prompts.
(add python-pdbtrack-track-stack-file to comint-output-filter-functions):
Tracking is plugged in to all comint buffers once python.el is loaded.
(python-pdbtrack-overlay-arrow): Toggle activation of
`python-pdbtrack-minor-mode-string' in addition to the overlay arrow.
(python-pdbtrack-track-stack-file): Use new
`python-pdbtrack-get-source-buffer' for more flexible access to
debugging source files.
(python-pdbtrack-get-source-buffer): Identify debugging target
buffer according to pdb stack trace, optionally using new
`python-pdbtrack-grub-for-buffer' if file is not locally
available.
(python-pdbtrack-grub-for-buffer): Find most recent python-mode
named buffer, or having function with indicated name.
(python-shell): Remove comint-output-filter-functions hook
addition, it's being done elsewhere.  Wrap long line.