[mainvolume] Add listening timer notifier and high volume step notifier.

Listening timer notifier reads configuration from /etc/pulse/mainvolume-listening-time-notifier.conf
by default. Module argument listening_time_notifier_conf overrides configuration file
location.

Listening time notifier configuration syntax:
timeout = timeout in minutes
sink-list = comma separated list of sinks that are watched for timer
mode-list = comma separated list of audio routing modes that are watched
for timer

When listening time notifier timeout expires, D-Bus signal is sent with
name 'NotifyListeningTime' with unsigned int argument 'ListeningTime'
containing timeout value in minutes.

High volume step notifier gets its value from algorithm parameters from
new property x-nemo.mainvolume.high-volume-step. This step is the first
volume step deemed harmful for hearing. If the property is not defined
high volume notification is disabled for given mode.

When volume step with same or higher value thatn high volume step is
selected, D-Bus signal is sent with name 'NotifyHighVolume' with
unsigned int argument 'SafeStep' with highest safe step value (which
means x-nemo.mainvolume.high-volume-step - 1).
[parameters] Add option for checking audio mode from other than voice module.