settings-widget: display unique aliases in the bluetooth device list

Adds a function that checks for uniqueness for a given bluetooth alias and generates a new alias for display (with postfix " (1)"," (2)". . ." (n)") if a duplicate alias is found.

This is built on top of c9adad45 and is another solution to !24. Ideally gnome-settings would be able to manage and save custom aliases for specific devices, but this is a simpler solution that just differentiates devices with unique aliases. Some additional testing and review would be great to make sure that the get_unique_alias function has no edge cases.