Fix the selection of instances

When a new instance was selected from the list of instances, it had no
effect because the new instance URL was saved in an attribute that was
reset at the next call of the add-on.
Now when the user selects a new instance, the associated setting is
updated so that this value can be reused the next time the add-on is
called or started.

Also took this opportunity to refactor the access to the add-on's
settings: there are now wrapper methods in kodi_utils.py which
encapsulates the call to Kodi APIs to make the code simpler.

See merge request StCyr/plugin.video.peertube!10 for more information

resources/lib/kodi_utils.py

@@ -8,6 +8,7 @@
     See LICENSE.txt for more information.
 """
 import xbmc
+import xbmcaddon
 import xbmcgui
 
 
@@ -21,13 +22,22 @@ def debug(message):
 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
+    :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
+    :return: Value of the window property
     :rtype: str
     """
     return xbmcgui.Window(10000).getProperty('peertube_{}'.format(name))
 
+def get_setting(setting_name):
+    """Retrieve the value of a setting
+
+    :param str setting_name: Name of the setting
+    :return: Value of the setting named setting_name
+    :rtype: str
+    """
+    return xbmcaddon.Addon().getSetting(setting_name)
+
 def notif_error(title, message):
     """Display a notification with the error icon
 
@@ -74,3 +84,11 @@ def set_property(name, value):
     :param str value: New value of the property
     """
     xbmcgui.Window(10000).setProperty('peertube_{}'.format(name), value)
+
+def set_setting(setting_name, setting_value):
+    """Modify the value of a setting
+
+    :param str setting_name: Name of the setting
+    :param str setting_value: New value of the setting
+    """
+    xbmcaddon.Addon().setSetting(setting_name, setting_value)