Add a KCM setting to control pulse audio module combine-sinks

Summary:
Add a KCM setting to control pulse audio module combine-sinks

This allows one to stream the same audio to multiple devices, for
example
the normal headphone port on a laptop and a USB device.

We use GConf rather than calling load_module directly as we need
pulseaudio to restore the settings after restart. There is a gconf
module
in pulseaudio already monitoring this setting.

GConf code is wrapped via a fork of a class from libqtgconf, but
modified
so that we only create one QObject per gconf subtree.

GConf code seting code is based on the code in paprefs.

Test Plan:
Clicked checkbox - ensured new output appeared
Confirmed checkbox initial state is correct
Confirmed toggling the setting in paprefs updates the checkbox in real
time

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2073