The Task Manager provides various QAbstractListModel-based model for listing Windows (TaskManager::AbstractWindowTasksModel), Startup tasks (TaskManager::StartupTasksModel) and Launcher Job (TaskManager::LauncherTasksModel).
libkworkspace provides functions to allow you to interact with the KDE session manager (SessionManagement).
Notification Manager Library
libnotificationmanager is responsible for listing notifications, closing them and interacting with them in Plasma. This class provides a Qt model for jobs: NotificationManager::JobsModel. As well as a Qt model for notifications and jobs: NotificationManager::Notifications.