I have not added all the options for this new tool yet, but this sets
the base. I also added a bit of TODO for several places where we need to
make it settable, in particular the fuzzy select tool, but also simply
PDB calls (this will need to be a PDB context settings.

Maybe also I will want to make some LineArtOptions struct in order not
to have infinite list of parameters to functions. And at some point, it
may also be worth splitting a bit process with other type of
selection/fill (since they barely share any settings anyway).

Finally I take the opportunity to document a little more the parameters
to gimp_lineart_close(), which can still be improved later (I should
have documented these straight away when I re-implemented this all from
G'Mic code, as I am a bit fuzzy on some details now and will need to
re-understand code).