Currently some classes like Callback are not extended from View.

This break things a litte bit as strict type checking is not possible. Is there any significant reason or can all UI elements than can be added to the render tree be based on the View class?

Afftected (top-most only) classes:

• atk4/ui/Callback
• atk4/ui/Template
• atk4\ui\TableColumn\Generic

Once fixed, change no class (unsafe) checking to check if the new objects extends at least self/View - see https://github.com/atk4/ui/pull/929/files#diff-248b7e363055cfdcca5a52260252ef41R425