Simplify display when there's only a single input device and a single output device

Summary: The Audio Volume plasmoid has always seemed a bit too heavy and complicated for the common use case of a single input device and a single output device, or apps playing audio but not also recording audio. This patch presents a simplified UI for those use cases.

Test Plan:
Before, Devices tab, single devices:

After, Devices tab, single devices:

After, Applications tab, only playback streams:

As I lack the hardware to test the multi-device use case, testing by others would be appreciated.

Reviewers: Plasma, drosca, VDG

Reviewed By: drosca

Subscribers: mart, Zren, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D11166