[Desktop Theme KCM] Adapt clock to new hand rotation center options

Summary:
The new hints

• hint-hourhand-rotation-center-offset
• hint-minutehand-rotation-center-offset

allow themes to define where in the hand pixmap the rotation center should
be placed.

Test Plan:
Clear the cache (rm .cache/plasma* -r) and restart system settings, see
clocks of unmodified themes still have hands at normal position, and hands
of modified themes at expected positions.

Reviewers: Plasma, VDG, mart, davidedmundson, ngraham

Reviewed By: Plasma, VDG, mart, ngraham

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D20113