Guide the user when libtorrent cannot be imported

Libtorrent is required to play videos but its installation is still
manual so now a message is displayed when libtorrent could not be
imported instead of having a "service could not start" error at Kodi
startup.
The message contains a link to a page which explains how to install
libtorrent. It will be displayed when:
* the add-on starts
* the user selects a video to play (including when called externally)

Other additions:
* Create a kodi_utils module to centralize some calls to the Kodi API
* Add license information in the header of the files
* Ignore some files in Git (python cache and Mac OS system file)

resources/lib/kodi_utils.py

@@ -0,0 +1,76 @@
+# -*- coding: utf-8 -*-
+"""
+    Utility functions to interact easily with Kodi
+
+    Copyright (C) 2021 Thomas Bétous
+
+    SPDX-License-Identifier: GPL-3.0-only
+    See LICENSE.txt for more information.
+"""
+import xbmc
+import xbmcgui
+
+
+def debug(message):
+    """Log a message in Kodi's log with the level xbmc.LOGDEBUG
+
+    :param str message: Message to log
+    """
+    xbmc.log(message, xbmc.LOGDEBUG)
+
+def get_property(name):
+    """Retrieve the value of a window property related to the add-on
+
+    :param str name: name of the property which value will be retrieved (the
+    actual name of the property is prefixed with "peertube_")
+    :return: the value of the window property
+    :rtype: str
+    """
+    return xbmcgui.Window(10000).getProperty('peertube_{}'.format(name))
+
+def notif_error(title, message):
+    """Display a notification with the error icon
+
+    :param str title: Title of the notification
+    :param str message: Message of the notification
+    """
+    xbmcgui.Dialog().notification(heading=title,
+                                  message=message,
+                                  icon=xbmcgui.NOTIFICATION_ERROR)
+
+def notif_info(title, message):
+    """Display a notification with the info icon
+
+    :param str title: Title of the notification
+    :param str message: Message of the notification
+    """
+    xbmcgui.Dialog().notification(heading=title,
+                                  message=message,
+                                  icon=xbmcgui.NOTIFICATION_INFO)
+
+def notif_warning(title, message):
+    """Display a notification with the warning icon
+
+    :param str title: Title of the notification
+    :param str message: Message of the notification
+    """
+    xbmcgui.Dialog().notification(heading=title,
+                                  message=message,
+                                  icon=xbmcgui.NOTIFICATION_WARNING)
+
+def open_dialog(title, message):
+    """Open a dialog box with an "OK" button
+
+    :param str title: Title of the box
+    :param str message: Message in the box
+    """
+    xbmcgui.Dialog().ok(heading=title, line1=message)
+
+def set_property(name, value):
+    """Modify the value of a window property related to the add-on
+
+    :param str name: Name of the property which value will be modified (the
+    actual name of the property is prefixed with "peertube_")
+    :param str value: New value of the property
+    """
+    xbmcgui.Window(10000).setProperty('peertube_{}'.format(name), value)