Merge branch 'gpointer/generalize'

This change allows gobjs to be pointed to by gpointers.  This makes it
possible to improve the interface for the *info objects, as they can pass
around lists of gpointers to refer to objects/canvases.  This is better than
using index numbers and canvas levels, which is cumbersome and in some cases
much slower (since a walk through the glist is required each time to find
the referrent for an index).