Add threshold setting in ES for battery warning

[cdeletre]

This PR adds the possibility to adjust the battery threshold in ES for the warnings.

The slide is displayed even if ENABLE AUDIBLE BATTERY WARNING is set to false because it the threshold is also used for the LED alert (which I wasn't able to test on RP5).

I've added this lines because I'm not 100% about the default behaviour when the value is not found in system.cfg after an upgrade.

	if (SystemConf::getInstance()->get("system.battery.warning_threshold").length() == 0) {
		SystemConf::getInstance()->set("system.battery.warning_threshold", "25");
	}