1) Added handling for no provider accounts configured (or enabled for incoming/outgoi...

1) Added handling for no provider accounts configured (or enabled for incoming/outgoing calls), and no
users configured (or enabled for outgoing calls).
2) Rewrote code to be more efficient by gathering all information about providers and users in one pass
at the start, and then using dictionaries with the data stored in them to build the sections.
3) Added translate() statements for default messages displayed (like "Dials all numbers").
4) Rewrote default messages for the "Incoming Calls" and "Providers Used for Outgoing Calls" sections
to be more descriptive.