A previous patch made the task manager only accept drag events which
contained URL lists or internal taskmanager mimedata in order to make it
easier to drag applets onto the panel.
This broke a feature where you can drag arbitrary mimedata between
windows and hovering above a task will raise that window.
This patch reverses the logic to only ignore plasmoids, and accept all
other drag/drops.
BUG: 375871
FIXED-IN: 5.9.2