implement text hint interface for automatic hover

Summary:
instead of manual invocation, re-use the text hint interface for auto-hover invocation
do manual hover the same way, just trigger the new functionality

it is a bit hacky, as the text hint interface isn't really made for async stuff,
but it works well ;=)

Reviewers: dhaumann, mnauwelaerts

Reviewed By: dhaumann, mnauwelaerts

Subscribers: brauch, kwrite-devel

Tags: Kate

Differential Revision: https://phabricator.kde.org/D22595