New camera selection API in QML.

Also added a new QtMultimedia global object which makes it possible
to retrieve the list of available cameras. It can be extended with
new utility functions in the future.

Includes documentation, example and auto tests.

Task-number: QTBUG-23770
Change-Id: Ifea076329c3582ea99246ee1131853344a7b773f
Reviewed-by: Christian Stromme <christian.stromme@digia.com>