Port agent to QML, and add an interface for Plasma Mobile

Replaces !12, !4

Based on mockups from: https://phabricator.kde.org/T8569

Will fix: teams/plasma-mobile/issues#106

![image](/uploads/9ebda04d4dcc3f0e3981517f188c9e80/image.png)

![image](/uploads/ddfb8af85be681ebbd4660a39c75d740/image.png)

OLD:

![image](/uploads/b4db4cff3f97302f3b67d2ef2463412c/image.png)