Add GimpGeglProceure to keep track of recently used GEGL operations in
the filter history. The new procedure also takes care of running the
op in the GEGL tool, so filters-commands.c is almost empty now.

Change gimp-filter-history.c to find procedures by name instead of
comparing pointers.

The only thing missing now is rerunning a GEGL op with the last
settings (not just showing its UI).