taskmanager: Add separator between app and Plasma actions

Inspired by and a complement to !133 (merged). This adds a separator to logically separate Plasma's actions with those provided by the application.

This helps when glancing at the items in the list, and should simplify the explanation to less tech-savvy users (if they ask) why an application may appear to have two "Open in New Window" options.

Before (5.23.4) and After