Improve contrast of preferences-system-time

Summary:
BUG: 390800

FIXED-IN: 5.55

Add outline to the preferences-system-time icon to improve contrast with the Breeze-dark desktop theme.

The outline is a bit small in the context menu, but that's due to this 32px icon being used at 16px size. Perhaps an action icon like clock should be used there instead.

Test Plan:
preferences-system-time:

Contrast with tooltip:

Contrast in context menu:

Reviewers: VDG, ngraham, ndavis

Reviewed By: VDG, ngraham, ndavis

Subscribers: ndavis, ngraham, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D18432