Introduce SettingState* elements to ease KCM writing
This is the QML based counterpart of D27540. Unlike with the KCModule
case, this is not automatically propagated to the ConfigModules, they
will all have to be adapted to make use of it.
I provide another patch which ports a few ConfigModule to see
how it looks: D27841.
Reviewed By: bport
Subscribers: broulik, kde-frameworks-devel
Differential Revision: https://phabricator.kde.org/D27840