Support creating custom treemodel

[daym]

it would be nice if one could create on'es own implementations of TreeModel in Guile. For this, it's especially necessary to be able to actually set the fields of a GtkTreeIter (which, for all other uses, are private).

I found register-type which one can use to register a new GObject descendant, but I haven't found how to specify which GInterface it implements nor how to implement the actual procedures a specific (gobject) interface requires.

[daym]

Either this or #95 is required to make it possible to use GtkTreeView to the full extent possible in C.