Move PixelFormat enum out of QVideoFrameFormat

The enum "PixelFormat" is defined inside the class QVideoFrameFormat. This enum is used by QCameraFormat in the public QML Camera API, but the class QVideoFrameFormat is not exposed to QML. This means we cannot expose the PixelFormat enum to QML, as long as QVideoFrameFormat is not meant to be exposed to QML.

We should move the PixelFormat enum definition out into a different namespace (for example, QtVideo), in a way that does not break binary compatibility and allows to us expose it to QML.