Use font: instead of font.pointSize: where possible

Summary:
Otherwise we're discarding style information possibly set in
the user's small font choices when we want to set a smaller
font.

Test Plan: Stuff that's suposta look small still looks small

Reviewers: cblack, Plasma

Reviewed By: cblack, Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D29438