[DMN Designer] Added item definition has not icon of structure

If user add custom item definition and define it as structure, the item definition misses the structure icon set the attached picture. There is new custom structure item definition a without icon. Compare it with other structure item definitions.

Acceptance test

• Check added item definition has icon when it is top level
• Check added item definition has icon when used in nested level
• Check item definition has not structure icon when changed from structure to something else
• Check item definition has structure icon when changed from something else to structure
• Check item definition has structure icon if item definition is recursive