Deprecate KService::pluginKeyword property
In most cases it is not needed anyways, because KPluginFactory can register multiple plugins with different base classes. Consequently there is no need for the keywords.
If there are multiple classes of a common base class registered they should be split up in separate libs.
Task: https://phabricator.kde.org/T14730
As mentioned in https://phabricator.kde.org/T14744 the KWin usages need to be ported, which is more complex. but the usages do not go through KService anyways and are consequently not blocking for this.