WIP: Add ONVIF motion detection support and subscription handling

[OttoHollmann]

As discussed in #173 current agent doesn't support onvif motion detection.

This PR adds a function that creates an ONVIF subscription and listens for new ONVIF events. If a message with the following content is received:

<tt:Data>
	<tt:SimpleItem Value="true" Name="IsMotion"/>
</tt:Data>

it should start recording. The recording should be stopped once the same message (with false) is received again:

<tt:Data>
	<tt:SimpleItem Value="false" Name="IsMotion"/>
</tt:Data>

Changes Overview:

• Make the topicKinds in the CreatePullPointSubscription function configurable.
• Add a new config option AGENT_CAPTURE_MOTION_ONVIF that will disable internal motion detection and use ONVIF events instead.
• Add functions for retrieving, parsing, and processing ONVIF motion messages.

TODO:

• Correctly handle start and stop recording
• Update the UI
• Handle configuration change/reload
• Update the documentation
• Feature is thoroughly tested