support push of single dict fields

[hhaensel]

This PR is the basis for pushing single fields of dictionaries and possibly array elements in the future.
I cleaned up the push!(model::M) methods for this purpose.

[hhaensel]

The current version now handles dicts and arrays

[hhaensel]

on() routines for array or dict fields that are expected to support direct element access should now be defined as

on(model.dict) do dict, _...
    # do something with dict
end 

or

on(model.dict) do dict, options...
    # do something with dict and options
    if isempty(options)
        # do the standard thing
    else
        v, jskeys = options[1:2]
        # do something with the changed value
end 

In the future this will probably change to rather return the object and the julia index (x, nestedkeys = options[1:2] or unnested = options[1]). That way we will be compatible if ever we move this interface to Observables.