[Task Manager] Add support for Unity Launcher API and Application Jobs

This adds support for the Unity Launcher API [1] with which applications can show a progress
indicator, a number badge as well as demand the user's attention. It also shows application
progress, such as copying a file, at the respective application entry.

[1] https://wiki.ubuntu.com/Unity/LauncherAPI

REVIEW: 126621
BUG: 343632
FIXED-IN: 5.6.0