KParts: add PartLoader as replacement to KMimeTypeTrader for parts

Summary:
It's based on the JSON metadata embedded into the plugins in kf5/parts/,
but it also has fallback code using KMimeTypeTrader internally until
KF6, so that we can start using PartLoader in applications.

Test Plan: Tested with partviewer, see also next commit for unittest

Reviewers: aacid, nicolasfella, kossebau

Subscribers: kde-frameworks-devel

Tags: Frameworks

Maniphest Tasks: T12173

Differential Revision: https://phabricator.kde.org/D27966