feat(kcm): zoom preview automatically

Summary:
On output size and position changes reset the relative output position in the
overview automatically.

This way the outputs are always movable with ease and the "center view" button
is not necessary anymore.

Test Plan: Moved outputs in multi monitor environment. Overview zoomed automatically.

Reviewers: #plasma

Subscribers: davidedmundson, ngraham, plasma-devel

Tags: #plasma

Maniphest Tasks: T11929

Differential Revision: https://phabricator.kde.org/D25027