🍒 applets/taskmanager: make no-image tooltip clickable

355e5a88 enabled basic tooltips when image previews are disabled. These "basic" tooltips still have most of the functions of the regular toolips with previews; they show each window of the task, allow users to close these windows, access media controls, etc. What's absent is maybe the most important function: focusing one of those windows. This is because that is done by clicking the thumbnail, and the thumbnail-less tooltips have no alternate method. This is unfortunate, and confusing as the often large areas look enticingly clickable.

This change makes the header clickable in case there are no thumbnails; the version with thumbnails should be unaffected.

(cherry picked from commit 357184c3)

Co-authored-by: Christoph Wolk cwo.kde@posteo.net