[Media controller] Add simple volume control

Summary:
Add simple volume control to Media Controller applet. Volume can by changed by:

- mouse wheel on applet icon (3 % step), and
- new global shortcuts (5 % step, unbound by default).

There is no mute support currently (mpris interface does not have a mute method so we would have to keep
track of old volume).

BUG: 386588

Test Plan: Tested in daily use with Cantata and SMPlayer. There is no UI change.

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: broulik, nicolasfella, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D11021