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: ahiemstra, apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D29437