Plasma::Theme: Allow packages to ship metadata.json file, move config to separate file

Currently we have both config and metadata in the metadata.desktop files.
This means we can not simply convert the desktop files to json,
but currently need to install the desktop files and convert them to json at runtime,
every time kpackage queries the respective packages

With this change, the config is written in the ´plasmarc` file and the metadata
can be in the json format.

Task: https://phabricator.kde.org/T15346