[tabbox] Add "HasIdenticalIconRole" to model to disambiguate identical icons

Certain TabBox visualizations use icons rather than thumbnails. In this mode, clients with the same icon (e.g. in the case of an app with multiple windows open) will be visually indistinguishable, and the user will have to tab over them and read their window titles to tell them apart.

This commit adds a new "HasIdenticalIconRole" to the TabBox client model, which allows visualizations to know when any given client has the same icon as one or more others, which it can use to show an appropriate disamgibuation UI so that the user can more easily distinguish them.

This new property is used in kdeplasma-addons!69 (closed).