Introduce AbstractAnnotationItemDelegate for more control by consumer

Summary:
The AbstractAnnotationItemDelegate is modelled after QAbstractItemDelegate
and should allow consumers of the annotation interfaces to customize the
rendering and the tooltip per annotation as wanted.

Reviewers: Kate, KDevelop, dhaumann

Reviewed By: Kate, dhaumann

Subscribers: cullmann, dhaumann, kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D8708