A feature that went missing in the transition from Plasma 4 to Plasma 5 was the ability to configure the icon widget. The upcoming Plasma 5.9 release is where this dialog will finally make its entry into the 5.x series.
In keeping with tradition of LTS aftermaths, the upcoming Plasma 5.9 release – the next feature release after our first Long Term Support Edition – will be packed with lots of goodies to help you get even more productive with Plasma!