Bug 721249 - pdb.gimp_context_set_brush_size() dont't work

Port selection/path stroking to using the PDB-controllable
paint options that live in GimpPDBContext.

Change gimp_item_stroke()'s "use_default_values" boolean which was
introduced at a time where we had no better way by a GimpPaintOptions
parameter. If a non-NULL paint_options is passed (when called from the
PDB), use it for stroking; if NULL is passed, use the actual paint
tool options from the GUI (when called from the menus or the stroke
dialog). In the PDB wrappers, get the right paint options object from
the PDB context associated with the calling plug-in.