[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