Battery: Handle Linux kernel events

The Kernel sends notifications for AC (un)plug and some
other important power supply events, so that we can
instantly update the widget. Apart from that it sends
notifications for any added or removed power supplies,
so that the battery support can be reinitialized (useful
on systems with removable batteries).

src/battery/battery.h

@@ -94,6 +94,7 @@ void default_battery();
 // freed memory
 void cleanup_battery();
 
+void update_battery_tick(void* arg);
 int update_battery();
 
 void init_battery();