[DMN Designer] FunctionGrid: Move F, J, P selector to header

The FunctionGrid allows switching between FEEL, Java and PMML function definitions using the context menu. etirelli has requested this be changed to a selector in the header. A new column needs to be added with the F, J, P selector in the new column header. The header should display F, J or P accordingly. Do not include row numbers (i.e. don't just add a RowNumberColumn!

Also, the default in classes should be String, not Undefined.

The cell that shows function parameters also needs to show the text "Select parameters" when there is none parameter added and the parameters when there is parameters.