Add a debug procedure group, living in 'debug.pdb', which would host
useful debug helper functions.  Functions in this group are not part
of the stable API, and may be changed at any point.

All procedures added to 'debug.pdb' should have a 'debug_' prefix,
and use the new std_pdb_debug() macro, which adds the proper "here be
dragons" warning to their description.

Add two debug procedures: gimp-debug-timer-start() and
gimp-debug-timer-end(), which measure elapsed time, a la
GIMP_TIMER_{START,END}, and can be used to profile script-fu
commands.