Instead, set their statically remembered pointer as a weak pointer so
it's nullified automatically if they are ever finalized. Doesn't
actually get rid of them upon exit yet, this is just a preparation.
I had to remove the asserrt in tool preset constructor, because with it
it wasnt possible to initalize an empty model object for the editor.
At init time the gimp object passed to the constructor is not yet available.
- add "Gimp" property to GimpToolPreset
- implement GimpConfigInterface::deserialize_property() and deserialize
the "tool-options" locally so we can pass the required "gimp" construct
property.