Create a PeerTube class to handle REST APIs
The PeerTube class will be responsible for providing methods to call easily the PeerTube REST APIs.
Other changes:
- the video filter is now also used when searching videos
- in case of error when sending a request, the message from the response is displayed even when listing the instances
- all the debug messages are now prefixed with the name of the add-on directly in kodi_utils: it allows an easier usage of this function anywhere in the add-on