applets/battery: indicate when manually set to balanced profile

When selecting balanced profile explicitly through the applet, as opposed to automatic switching due to AC/battery/low battery profile, this means a special state, so indicate this with an icon and show the applet as relevant.

How to test: In power management settings, select switching to performance power profile on AC. Then, while on AC, select balanced profile in the applet. The applet should show up as relevant with the speedometer icon. To compare, select switching to balanced power profile on AC and leave it at balanced. The applet should remain quiet about this.