[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