Draft: sheet: Add a button to start a phone call

evolution-data-server provides a class E.PhoneNumber which -amongst other things- allows us to parse freeform strings as phone numbers. If successful, it also allows to generate a tel: URI which we can use to start a call with the user's preferred application. Note however that it's only really supported if E-D-S was compiled with support for libphonenumber (which provides the underlying implementation).

Marking as a Draft since I have some remarks that need to be answered first:

• From a technical PoV:
  • for some reason, the default country code for my contacts is set to the US country code (+1). I have no idea how to change this, from the CLI or from a Settings UI. We could add a preference in Contacts, but I'd like to have something that applies system-wide, so it works for Contacts, but also Evolution, Calls, etc.
  • Maybe this is something that can/should add in GNOME Settings?
• From a designers PoV:
  • how do we want to display telephone numbers that were given as freeform text?
  • Do we display the original phone number or the one with a fixed structure?
  • What if the parsed phone number adds a surprising country code (which could mean you accidentally make an international call)?