applets/taskmanager: don't time out preview tooltips

In general we don't time out any tooltips now as long as the cursor
remains hovered over the thing that opened the tooltip. One exception is
Task Manager window thumbnail tooltips. This may simply have been missed
during the change to no longer time out any tooltips (it has a local
override for the new default behavior) or it might have been
intentional at the time. But either way, it's probably better to follow
the default settings here for reasons of consistency, not to mention to
make it easier to interact with the preview thumbnails, especially when
there are multiple windows open.

BUG: 470043
FIXED-IN: 5.27.6