Add initial thermal view

Just thermal zone temperature for now, no cooling devices

This is just a quick forward port of the Purism's downstream work to newer libhandy and the new upstream codebase. We likely want to

• allow to use libgtherm as a subproject
• add cooling cells - though this needs gtherm improvements and could be done as a follow up
• add a user friendly mapping for well known thermal zone names (e.g. to separte gpu / vpu / cpu)

Fixes parts of #47