[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:
{F8125148}

After:
{F8125149}

Reviewers: #plasma, #vdg, drosca, ngraham

Reviewed By: #vdg, ngraham

Subscribers: ngraham, apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27577