[DMN Designer] Invocation: Improve header

Requested by etirelli:

On the function invocation, it is not clear where and how to set the function name for the invocation.

The header has two empty cells; the top right should not even be there and the other is where the function name should be entered. Think about changing the renderer to more correctly render the header - according to the DMN specification - and put a hint/place-holder for where the function should be entered.