Instantiate KPropertyWidgetsFactoryManager from KPropertyFactoryManager

Summary:

• this fixes missing instantiation of KDefaultPropertyFactory, now composed properties such as QSize work again
• make KDefaultPropertyFactory owned by KPropertyFactoryManager
• instantiation is completely delayed
• KPropertyFactoryManager::addInitFunction() can be used in the future for similar needs

Test Plan: Run kpropertyexample, QSize works, etc.

Reviewers: piggz

Differential Revision: https://phabricator.kde.org/D633