Locations: Add Bhutan locations and airports

Bhutan was missing information for its airports and some cities. I'm not totally sure if this is the right way to fix missing information, it seems some of the instructions regarding Locations.xml and sources/ are a bit outdated (still mention Locations.xml.in).

Is this correct? Also: What's the criteria for which cities to include?

Closes: gnome-clocks#4 (closed)