Add migration script for the digital clock applet
This PR adds a migration script to support a new setting on the digital clock applet. The old name for the config key didn't work as it was too specific: now you can switch between the timezone code, the city name and the UTC offset, whilst the config setting only mentioned the timezone code.
I'm not sure if the config file I'm reading is the right one, so please do let me know if I made a mistake.
Related PR: plasma-workspace!751 (merged)