Promote KCM to top level and rename accordingly

Summary:
Now that the Phonon KCM has been removed, there's no longer a need to put the plasma-pa
KCM inside the Multimedia category, since that category now only includes one item.

This patch promotes the KCM to be visible on the top-level in the Hardware category, and
tweaks its name and tooltip text to reflect how it now manages all audio devices.

Test Plan:

Reviewers: VDG, Plasma, drosca, nicolasfella, sitter

Reviewed By: nicolasfella

Subscribers: ndavis, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D23542