Status area: introduce battery & power indicator

Add an indicator for battery charge and power indication in the
system status area, using the newly exported DBus API from
gnome-power-manager.

https://bugzilla.gnome.org/show_bug.cgi?id=622451

js/ui/panel.js

@@ -33,6 +33,7 @@ const STANDARD_TRAY_ICON_ORDER = ['a11y', 'display', 'keyboard', 'volume', 'blue
 const STANDARD_TRAY_ICON_SHELL_IMPLEMENTATION = {
     'a11y': imports.ui.status.accessibility.ATIndicator,
     'volume': imports.ui.status.volume.Indicator,
+    'battery': imports.ui.status.power.Indicator
 };
 
 const CLOCK_FORMAT_KEY        = 'format';