Track if a thermostat's values are "dirty"/unsynchronized #7

When setting any attribute value on a thermostat, you need to do a write in order to push this value back to the thermostat. It'd be nice to know if there are any values that need to be written.

This would be written to the ~/.eco2.xml file by Thermostats.cs.

Being able to just do a write to all thermostats with "dirty values" at once would also be fantastic!