Allow to set maximum volume < 100%

Summary:
Currently one cannot set the maximum volume < 100%. This feature is required because some earphones / speakers are more sensitive than others and may get damaged at high volumes. In particular, one easily ends up overshooting safe volume levels when using the mouse scroll wheel to change the volume. After this change, one can safely use the mouse wheel to change volume.

BUG: 404387
FIXED-IN: 5.17.0

Test Plan:

1. Right-click on applet > 'Configure audio volume'.
2. Try to set maximum volume < 100% and click 'OK'.
3. Scroill on the systray icon to change maximum volume. Check if it is limited to the configured maximum.

Reviewers: Plasma, drosca, ngraham, romangg

Reviewed By: Plasma, ngraham, romangg

Subscribers: romangg, ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D23082