[Calendar] Adjust the calendar toolbar anchors

Summary:
Horizontally align the toolbar row to the days/month/etc grid canvas.

(The bar was used to be a few pixels wider)

Test Plan:
Before:

After:

Reviewers: Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: aacid, davidedmundson, broulik, plasma-devel, Plasma, Frameworks

Tags: Frameworks, Plasma

Maniphest Tasks: T7499

Differential Revision: https://phabricator.kde.org/D9019