Knotify implementation to reload plugins/Runner configurations

This is the counterpart of plasma/plasma-desktop!18 (merged) and fixes BUG 421426.

To test this in KRunner/the plasmoid you have to enable the watcher in the milou project:

RunnerResultsModel::RunnerResultsModel(QObject *parent)
     : QAbstractItemModel(parent)
     , m_manager(new RunnerManager(this))
 {
+       m_manager->enableKNotifyPluginWatcher();

This also works when you edit the config with the notify flag, example of doing this inside of a runner:

auto cfg = config();
cfg.writeEntry("test", "test", KConfig::Notify);
cfg.sync();

With the counterpart you can enable/disable a plugin in the runners KCM and the runners get reloaded when you click apply.

Info: I intentionally didn't enable this by default, because for example the Kickoff fronted doesn't need this feature (the runners are whitelisted and don't depend on the config).