desktoptojson: also convert InitialPreference key

Summary:
This is going to be useful to load KParts using KPluginLoader.
Most other plugins are simply all loaded, rather than selecting the
preferred one, which is why this hadn't been done until now.

Test Plan:
qtplugininfo $KDEDIR/lib64/plugins/kf5/parts/gvpart.so
(gwenview) now has InitialPreference under KPlugin rather than outside it.

Reviewers: arichardson

Subscribers: kde-frameworks-devel

Tags: Frameworks

Maniphest Tasks: T12340, T12173

Differential Revision: https://phabricator.kde.org/D26442