[Trac #555] Add PM event for battery status change

[Zarro Boogs per Child]

#555: Add PM event for battery status change
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 Reporter:  krstic       |       Owner:  mfoster
     Type:  enhancement  |      Status:  new    
 Priority:  normal       |   Milestone:  BTest-2
Component:  hardware     |    Keywords:         
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 We should be emitting an automatic PM event indicating the current battery
 charge percentage, either  on a periodic basis (e.g. every 1 minute) or
 every time the percentage changes.

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Ticket URL: <http://dev.laptop.org/ticket/555>
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