[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