This API gives the names of available backends and provides a basic
information about features/protocols supported by those backends.
Also, it has the 'loadBackend' functions which allow to select
a particular backend (which are becoming plugins).
At the moment, the implementation is still 'hardcoded', the
follow-up patch will allow to select different backends in runtime.
Task-number: QTBUG-65922
Change-Id: I05877de9c02857594e76b24d52e7578bdb01df69
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
ee89f3a6e3