Requiring tool definitions to call a function in _mcl_autogroup means
that they need to be loaded after _mcl_autogroup. This can cause
problems because _mcl_autogroup needs to be loaded after all tool and
node definitions are completed.
Because of this the API is changed so tool definitions instead put the
data used to generate groupcaps in the custom field
_mcl_autogroup_groupcaps. _mcl_autogroup will then go through all
registered tools and update the groupcaps for every tool where this
field is present. This means that no mod ever has to depend on
_mcl_autogroup.
