Improvements to Stub<T>

[egil]

Improvements to Stub<T>:

• Make it easy to invoke event callbacks passed to the stub
• Make it easy to render a render fragment or template passed to a stub
• Make it possible to use parameter data in custom markup render fragments.
• Add documentation about Stub<T>

[egil]

I'll close this. If these enhancements gets requested (aka folks dont just use a mocking framework), we can revisit these.