si-convert: watch links to the convert node instead of the node's state

there are underlying issues with the state management of the graph,
so it's not a very good idea to rely on it to activate / deactivate
our convert node depending on the links that exist

instead, track the links ourselves and create/destroy the corresponding
links to the target node accordingly; it's a more robust approach

it was also not entirely correct that we would previously configure
the convert node to be a driver (node.driver=true); maybe this caused
the underlying issues in the first place... we don't need it now
anyway, so it's gone