[taskmanager] Add filter for task role IsHidden, port requestToggleMinimized to use it
Requires and includes @abhijeetviswa's commit from !563 (merged).
Needed to make panel Adaptive Transparency work with "Show window thumbnails: always". I'll post an MR for that in plasma-desktop. Done.
Needed to make Minimize All work with "Show window thumbnails: always".
Required by plasma-desktop!457 (merged) and plasma-desktop!459 (merged).
Edited by Bharadwaj Raju