Activate minimise all plasmoid on mouse enter if the user is dragging mimedata

Summary:
This allows the user to easily drag a file from dolphin into a
folderview on a desktop even if windows are in the way.

BUG: 376212

Test Plan:
Dragged an item, things minimised
Normal mouse enter, nothing happened
Click still works as normal

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4521