Reverse TaskManager DragDrop to blacklist Plasma instead of whitelisting URLs

Summary:
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

Test Plan:
Dragged applet from Widget explorer onto task manager and the task
manager moved to
the side to allow a drop.

set sorting to manual and moved a task

Dragged some text onto a task, and that window raised immediately

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D4476