Use Qt Quick Controls 2 in the basic example

- Replace custom text input types with corresponding
  Qt Quick Controls 2 types.
- Replace custom scrollbar with ScrollBar from Qt Quick Controls 2.
- Only build the basic example if Qt Quick Controls 2 is available.
- Update the documentation and its screenshot.

Change-Id: Ib2559595d4a589bb228450bc08be5ab6fbc69b91
Reviewed-by: Jarkko Koivikko <jarkko.koivikko@code-q.fi>
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>