Guide the user when libtorrent cannot be imported

The goal of this feature is to make the installation of the add-on easier. Libtorrent is required to play videos but currently it must be installed manually on the machine.
Consequently when libtorrent cannot be imported, a message is displayed which gives instructions to the user on how to install it.
Moreover the service will not report an error anymore when libtorrent cannot be imported: when the add-on is installed automatically as a dependency of another add-on it will avoid bother the user with an error at each startup.

Also create a kodi_utils module to centralize some calls to the Kodi API: the logging functions, the access to the window properties and the notification.