Better display on list of objects in `print_traits`

Here is a minimal script to demonstrate the issue
```
from traits.api import HasTraits, List, Instance

class Child(HasTraits):
pass

class Parent(HasTraits):
children = List(Instance(Child))

if __name__ == '__main__':
children = [Child() for i in range(10)]
parent = Parent(children=children)
parent.print_traits()
```
which outputs:
```
children: [<__main__.Child object at 0x11dd...n__.Child object at 0x11dd0db48>]
```

The way it is abbreviated might be confusing such that one might misunderstand it as a list of single object. Might better try to add some informative suffix such as " and 9 other entries"