The GtkTreeModelIface::get_path() implementation was always returning
NULL, due to comparing the wrong pointer while trying to find the match.
This didn't appear to cause big problems in GTK 2, as the vfunc just
wasn't called in usual situations. However in GTK 3, at least since
upstream commit e53dc3479f8 ("Added GtkCellAreaClass->apply_attributes()")
it is called in the main code paths, and failure to return the correct
path results in editable cell renderers not knowing what they are editing,
throwing critical warnings on loading and crashing when something gets
edited.

This fixes Task Properties edit dialog "Resources" and "Predecessors" tabs
tree views, and probably more.