Commit usability changes by Dennis Nienhüser and myself to the accounts and identity config:

*) change identityId to id for clarity
*) use an immutable id as an internal handle
*) introduce user-editable label field
*) use pointers for default identity

*) replace new identity source dialog with Add and Copy buttons
*) don't show identity selector dialog on switch when there are only 2
identities
*) adapt dbus interface to use ids, map these to labels
*) more string tweaks
*) order the KCM pages using X-KDE-Weight