Add sound warning for low battery state (easy)

[madmax]

As ​discussed in the forums, some users may want a loud sound warning on top of the visual notification when the battery goes below certain charge level. The notification is currently created by PowerStatus in ​_NotifyLowBattery. At first sight, it should be simple to use ​add_system_event_beep to define a sound event for this case and ​system_beep to use it.

[Handmaus]

Copypaste of the suggestion I made in the forum thread, for the record:

I imagine an entry in the menu labeled 'Sound warning...' that opens a little panel that lets you set the alarm, a slider to set the battery percentage that triggers it (or percentages, maybe you want to charge that battery to just, say, 80%, because you don't plan to use it for some months), and a button labeled 'Set sound' that opens the Sounds panel.

[Handmaus]

Just to clarify, in the previous comment, where it mentions "...an entry in the menu...", I was referring to PowerStatus applet's contextual menu.

[twang12-cpu]

If this is not already being worked on, I would like to work on this.

[waddlesplash]

Done in hrev57123.