This adds syntax highlighting for the notebook debugger of ticket #7509.

Depends on patch attachment: trac_7509.5.patch:ticket:7509.

For reasons of speed only a small part of the source code around the current position is loaded and highlighted. We detect when the
code fragment starts in the middle of a docstring to ensure correct
highlighting in this case.

CC: @fchapoton

Component: notebook

Author: Wilfried Huss

Issue created by migration from https://trac.sagemath.org/ticket/12451