libobs: Add data signal to audio sources

This adds a new signal to (audio) sources which is emitted whenever new
audio data is received from the source. This enables other code that is
interested in the raw audio data to directly access it when it becomes
available.
libobs: Add update interval property to volmeter

Add a property to the volume meter that specifies the length of the
interval in which the audio data should be sampled before the
audio_levels signal is emitted.
libobs: Add peak hold property to volmeter

Add a property to the volume meter that specifies the time for which
peak value should be held until it is reset.
libobs: Add level calculations to volmeter

Use the new audio_data signal from sources to calculate the volume
levels in the volume meter.
libobs: Remove audio levels handler from volmeter

Remove the the signal handler for the volume_level signal of audio
sources from the volume meter in anticipation of using the levels
calculated in the volume meter itself.
libobs: Signal own levels in volmeter

Signal updated volume levels when they become available in the volume
meter. The frequency of the updates can be adjusted by setting a
different update interval.
libobs: Remove volume level signal from obs_source

Remove the calculation of volume levels and the corresponding signal
from obs_source since this is now handled in the volume meter.
Code that is interested in the volume levels can either use the
volmeter provided from obs_audio_controls or use the audio_data signal
to gain access to the raw audio data.
obs: Remove update time from volume meter

Remove the update time check from the volume meter since it is no longer
nescessary. The update interval can now be set with the volmeter which has
the added benefit of not loosing data where previously updates had been
skipped.
Merge pull request #311 from fryshorts/audio-analyzer-move

libobs: Move audio level calculations