EventIcon: only trigger 'activate' signal on principal button click

With the introduction of 'activate' signal on [1], and the use of that
signal in Sugar Home, we introduced a bug, now, pressing the secondary button
starts the activity instead of open the palette.
This patch solves the issue checking the putton pressed before send the signal.

[1] a19cf9ed27

src/sugar3/graphics/icon.py

@@ -762,6 +762,7 @@ class EventIcon(Gtk.EventBox):
         self.set_palette(Palette(text))
 
     def __button_release_event_cb(self, icon, event):
+        if event.button == 1:
             alloc = self.get_allocation()
             if 0 < event.x < alloc.width and 0 < event.y < alloc.height:
                 self.emit('activate')