[DMN] Move the "structure" option to top of the Data Type drop-down

We received usability feedback that the Structure option is difficult to find. To improve usability it was suggested to move that option to the top of the dropdown list, and if possible add a live search option. 

Example of the issue, using dmn.new sample file. Notice how the default option of "Structure" is difficult to find. Additionally, it is not co-located with the Default options.