D29436: Use theme.smallestFont instead of various ad-hoc alternatives

[Nathaniel Graham]

ngraham added a reviewer: cblack.

REPOSITORY
  R242 Plasma Framework (Library)

REVISION DETAIL
  https://phabricator.kde.org/D29436

To: ngraham, #plasma, cblack
Cc: kde-frameworks-devel, LeGast00n, cblack, michaelh, ngraham, bruns