[appindicator] use gir package name "AppIndicator3"

Allow to additionally import "AppIndicator3" to "AyatanaAppIndicator3"

appindicator_integration/plugin.py

@@ -14,11 +14,16 @@ import gi
 from gi.repository import Gtk, GLib, Gdk
 
 try:
-    gi.require_version('AyatanaAppIndicator3', '0.1')
-    from gi.repository import AyatanaAppIndicator3 as appindicator
+    gi.require_version('AppIndicator3', '0.1')
+    from gi.repository import AppIndicator3 as appindicator
     ERRORMSG = None
 except (ValueError, ImportError):
-    ERRORMSG = 'Please install libappindicator3'
+    try:
+        gi.require_version('AyatanaAppIndicator3', '0.1')
+        from gi.repository import AyatanaAppIndicator3 as appindicator
+        ERRORMSG = None
+    except (ValueError, ImportError):
+        ERRORMSG = 'Please install libappindicator3'
 
 from gajim.common import app, ged
 from gajim.common import configpaths