Clicking outside preview box moves view to that point

Summary:
Clicking outside of the preview box in the overview docker now moves the view of the canvas to be centered at that point.

BUG: 384687

Patch by Seoras Macdonald

Reviewers: #krita, dkazakov
Reviewed By: #krita, dkazakov
Subscribers: dkazakov, emmetoneill
Differential Revision: https://phabricator.kde.org/D12178

(cherry picked from commit 0eb11f4eaf8439a675fa41a08d62fc5c75fbba8c)