Move PimCommon::GenericGrantleeFormatter here

Summary:
This allows us to use the i18n and icon loading integration in there.
Apart from adding that, there have been small API cleanups (such as no
longer inheriting from QObject) since we need to adjust all users anyway
now.

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D20988