Implemented aspect ratio and scale mode handling in Phonon MMF backend

- Replaced VolumeObserver and VideoOutputObserver interfaces with
signals.

- Added signals for propagating changes in aspect ratio and scale mode
from VideoOutput to VideoPlayer.

- Removed VideoPlayer::getNativeWindowSystemHandles.  Interaction with
window system is moved into VideoOutput, so that VideoPlayer is better
focussed on its main task: interacting with CVideoPlayerUtility.

- WId changes, resize and move events received by the VideoOutput
widget cause it to emit a videoWindowChanged signal.  This is received
by the VideoPlayer, which triggers a call to updateVideoRect.  The
main task of this function is to calculate the scale factors which are
provided to the MMF via CVideoPlayerUtility::SetScaleFactorL.  Note
that:
	i) This function must be called both before and after the call
	to SetDisplayWindowL.  For changes in aspect ratio or scale
	mode, setting the scale mode after updating the display window
	is sufficient.  However, testing showed that, when switching in
	or out of full-screen mode, two calls were necessary in order
	to preserve the correct aspect ratio.

	ii) The screen rectangle passed to the MMF is still the full
	window extent; it is not the region in which video will
	actually be rendered.  The post-processor will fill in the
	remainder of the window with a background colour (typically
	black).  If, on the other hand, we passed in the actual video
	display rectangle, we would need to do this background painting
	in the widget.  This in turn would require a change to QtGui:
	at present, we can only disable blitting on a per-widget basis
	(by setting QWExtra::disableBlit).  If we needed to paint the
	borders of the video window, disableBlit would need to contain
	the actual DSA region, rather than just a single boolean flag.

Task-number: QTBUG-5585
Reviewed-by: Frans Englich

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