add settings if default value for a field is given

[rallek]

You have already this great settings for the images. Similar to this you may implement it for all fields where a default value is created within the model.

I would prefere this instead of #263.

This feature is not requested for 0.7.0

[Guite]

Wouldn't this result in a huge amount of setting fields... not sure whether this would be really useful...

[Portugao]

Sometimes this feature is useful. So it would be fine to have the option to activate it for each field; by default it could be inactive.

[Guite]

Note there is now an entity initialiser class (see #995) which allows to dynamically apply default values for newly created entities. This might be helpful for solving this issue, too.

[Guite]

Postponing, because this depends on #97.

[Guite]

As agreed at the camp we are going to utilise expressions to let the default value of an entity field point to / reference a variable field.