popupMenu: Use an image for arrows

Until now the arrows were the associated arrow
character of the font. This cause some problems like
different arrows for different fonts, and size can be
altered because of the font size.
To solve that, use an image for the arrows.

https://bugzilla.gnome.org/show_bug.cgi?id=720206

js/ui/status/keyboard.js

@@ -341,7 +341,7 @@ const InputSourceIndicator = new Lang.Class({
 
         this._hbox = new St.BoxLayout({ style_class: 'panel-status-menu-box' });
         this._hbox.add_child(this._container);
-        this._hbox.add_child(PopupMenu.unicodeArrow(St.Side.BOTTOM));
+        this._hbox.add_child(PopupMenu.arrowIcon(St.Side.BOTTOM));
 
         this.actor.add_child(this._hbox);
         this.actor.add_style_class_name('panel-status-button');