Port from GConf to GSettings

Summary:
CCBUG: 386665

As discussed in bug 386665 GConf is deprecated and needs to be replaced by GSettings to keep features enabled.

Question: Do we need GConf as a fallback for PA versions without module-gsettings?

Test Plan:
Checkboxes in Advanced tab are enabled again.
Changed settings are shown in dconf-editor and vice versa.
Combine output checkbox shows/hides combined sink in applet

Reviewers: drosca, davidedmundson

Reviewed By: drosca

Subscribers: pino, lbeltrame, evpokp, rikmills, broulik, asturmlechner, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D14147