Check icon positions after move

Summary:
BUG: 402574

When we're moving a bunch of icons to the area which is already busy, we didn't check whether we're moving icons to different places.

Now we check additionally if we're moving icon to the place already taken by previously moved icons.

Reviewers: #plasma, hein

Reviewed By: #plasma, hein

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D17809