[Blockly][Codegenerator] functions that return values

[PaulaScharf]

Block description

Not sure if I am misunderstanding, but the return value of function is not usable at the moment.
For example I would like to use the return value of function "a", but its not exposed:

Expected block behaviour

Function calls that return a value should behave like a value. Also, I guess it would be nice if the function name would be a dropdown.