Prettify the Accounts KCM and expand KAccounts integration library

Summary:
This patch represents an effort which aims to expand, beautify, and clean the
KAccounts integration code and visuals. This builds on the existing work, by adding
new models and new job classes to the library, and exposing them through
the QML module, which is used in the KCM as seen in the screenshots below.

New functionality in the library:

AccountsModel
- A model listing what accounts are currently created on a user's system
ServicesModel
- A model listing what services (if any) are available in an account
ProvidersModel
- A model listing what providers are available for new accounts
ChangeAccountDisplayNameJob
- A job which allows changing the user visible name of an account
RemoveAccountJob
- A job which removes an account (and all its credentials)

The QML module has been modified to both include the old names of older jobs,
as well as the new, more descriptive names.

As for the KCM, please see the screenshots in the differential revision below
for what features have been added and modified (the above code all supports
the functionality in the KCM)

Committed with history on request

Differential Revision: https://phabricator.kde.org/D27681