Make task manager ignore events it can't handle

Summary:
Currently if I drag an item from widget explorer onto the panel and
hover over most applets the applet will shift showing me a drop
indicator when I can add the applet. However if I move over the task
manager this doesn't work.

This patch makes the taskmanager not intercept drop events that are not
related to the task manager, URLs or internal drops, so that adding
applets works correctly.

Test Plan:
Dropped a file on kate, still opened
Set ordering to manual, still could shuffle things
Dragged an item from widget explorer over the task manager, it
showed a drop indicator either side and appeared there when dropped.

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D3195