[Applet] Context Menu per device/stream to offer additional functionality
Adds a button in the applet to open a context menu, currently offering:
- Mute/unmute device/stream
- Set device as new default
- Change active port of device
Reviewers: drosca
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D4751