Simplify GenericGrantleeFormatter

Summary:
Mainly removing unneeded indirections to make this easier to read. The
next step is then moving this to grantleethemes so we can use the i18n
and icon integration for Grantlee here.

Reviewers: knauss, mlaurent

Reviewed By: knauss, mlaurent

Subscribers: knauss, kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D20793