Introduce Enum for domains, and lots more

[cyclops1982]

Ok, this is an ugly one, but does a lot:

• Make HAEntity aware of it's domain.
• Allow us to get the services /from/ the haentity. Some HAEntities do not have services (sensors for example)
• Cleanup the use of entries in the ftxui, as we now have a map<string,haentity> within the backend and can use that.
• REmove HABackend::GetServicesForDomain() as we should be asking that to the HAEntity, or to the HADomain.
• Make the backend get the services (to build up domains) first, before getting states and start listening to events.
• Let HABAckend::start() wait until we have loaded the information before continuing.
• Move external code (argparse, magic_enum) to ext directory and exclude this from clang-format.