[gnome-music] songwidget: Use accessor for event button

[Marinus Schraal <mschraal src gnome org>]

commit f6f2c82ca034e067e99f66b95d59a142ae314b59
Author: Tyler Garcia <tyler tylergarcia net>
Date:   Fri Aug 10 22:32:57 2018 -0700

    songwidget: Use accessor for event button
    
    Event fields are deprecated
    
    Closes: #213

 gnomemusic/widgets/songwidget.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/gnomemusic/widgets/songwidget.py b/gnomemusic/widgets/songwidget.py
index 3dfa792c..c4b2e434 100644
--- a/gnomemusic/widgets/songwidget.py
+++ b/gnomemusic/widgets/songwidget.py
@@ -133,7 +133,8 @@ class SongWidget(Gtk.EventBox):
     @Gtk.Template.Callback()
     @log
     def _on_star_toggle(self, widget, event):
-        if event.button != Gdk.BUTTON_PRIMARY:
+        (_, button) = event.get_button()
+        if button != Gdk.BUTTON_PRIMARY:
             return False
 
         favorite = not self._star_image.favorite