[KCM] Rework design and structure
This patch changes no functional stuff,
only the arrangement and headings of the controls.
In the end the KCM:
- wastes less space
- resembles more the applet
- has only 3 tabs instead of 5 anymore
- has a less complicated code structure
Reviewers: drosca
Subscribers: plasma-devel
Differential Revision: https://phabricator.kde.org/D4732