Control panel handle fields - add button to regenerate/refresh handle

[piotrpog]

Right now handle is automatically generated from title/label of the object. For example when creating field or entry type. Handle is updated as user types in the label, until handle field itself is focused on.

It would be sometimes useful to "refresh" handle field automatically, based on label changes that were done later. Some kind of small button placed next to handle field could be used for that.