[KCM]Fix content below scrollbars
Summary:
Set content to use available space.
Make scrollbars touch the side of the frame.
Fit speaker test grid inside the scrollview area.
Fix advanced content layout.
Make Header always fit inside the scrollview area.
BUG:416331
BUG:417447
Test Plan:
Open audio settings from kickoff.
Before:
After:
Reviewers: Plasma, VDG, drosca, ngraham
Reviewed By: VDG, ngraham
Subscribers: ngraham, apol, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D27577