This commit greatly improves the device specific settings support

The old device specific settings were difficult to understand and
not very clear in their naming. This commit drops the settings
object you had to add to each device to change it's settings.
Instead, all device specific settings are now just parameters just
like the device it's own parameters.

Old situation:

"rpi": {
	"name": "Rasp. Temp.",
	"protocol": [ "rpi_temp" ],
	"id": [{
		"id": 1
	}],
	"temperature": 43312,
	"settings": {
		"interval": 10
	}
}

New situation:

"rpi": {
	"name": "Rasp. Temp.",
	"protocol": [ "rpi_temp" ],
	"id": [{
		"id": 1
	}],
	"temperature": 43312,
	"poll-interval": 10,
	"gui-decimals": 2
}

The following new settings are supported:

Weather Stations / Sensors

- device-decimals
- device-temperature-offset
- device-humidity-offset
- poll-interval
- gui-decimals
- gui-show-temperature
- gui-show-humidity
- gui-show-battery

Contacts / Switches / Dimmers / Screens

- gui-readonly

Dimmers

- dimlevel-maximum
- dimlevel-minimum