calendar: Add calendar icon to events

Without the boxy background, event messages look a bit plain and
unaligned with other messages. Adding an icon addresses this,
however as repeating the same icon over and over again in case of
many events would be rather noisy, only show it for the top event
as in the mockups.

https://bugzilla.gnome.org/show_bug.cgi?id=775763

js/ui/calendar.js

@@ -706,6 +706,14 @@ const EventMessage = new Lang.Class({
         this._date = date;
 
         this.parent(this._formatEventTime(), event.summary);
+
+        this._icon = new St.Icon({ icon_name: 'x-office-calendar-symbolic' });
+        this.setIcon(this._icon);
+
+        this.actor.connect('style-changed', () => {
+            let iconVisible = this.actor.get_parent().has_style_pseudo_class('first-child');
+            this._icon.opacity = (iconVisible ? 255 : 0);
+        });
     },
 
     _formatEventTime: function() {