New QCameraInfo class.

New QCameraInfo class.

The class allows to get the list of available cameras on the system as
well as getting some static information about them such as their unique
ID, display name, physical position and sensor orientation.

This makes QCamera::availableDevices() and QCamera::deviceDescription()
obsolete.

This patch contains the API, documentation and auto-tests but not the
actual implementation by each backend (except for retrieving the default
camera device).

[ChangeLog][QtMultimedia] Added new QCameraInfo class
[ChangeLog][QtMultimedia] QCamera: availableDevices() and
deviceDescription() are deprecated, use QCameraInfo instead

Change-Id: I64fd65729ab26a789468979ed5444ee90bb82cd0
Reviewed-by: Christian Stromme <christian.stromme@digia.com>