Rename method name() to id()

Summary:
This makes sense because a string returned by this method is literally a
unique identifier that can be used either on a server side to identify a
data source or in an app itself for a lot of different purposes, e.g.,
for mapping data sources or performing some fast lookup by ID.
A method for getting a short user-visible name will be introduced in one
of the further revisions.

Reviewers: vkrause

Differential Revision: https://phabricator.kde.org/D19727