GStreamer: each QMediaPlayer now has its own volume by default.

We were using the volume property of GStreamer's playbin element to set
the volume. This could behave differently depending on the system
configuration; it could affect the system volume or the media player
own audio stream.
We now use a 'volume' element to do software attenuation on the audio
data sent to the audio sink, it allows each QMediaPlayer to always have
its own volume.
To preserve the previous behavior, developers can set the
QT_GSTREAMER_USE_PLAYBIN_VOLUME environment variable to true.

[ChangeLog][QtMultimedia][GStreamer] QMediaPlayer::setVolume() doesn't
change the system volume anymore (it could be the case before depending
on the system configuration). Set the QT_GSTREAMER_USE_PLAYBIN_VOLUME
environment variable to true to restore that behavior.

Task-number: QTBUG-30317
Task-number: QTBUG-36511

Change-Id: Ia0249962a74ac21fb110fcb634c08706f8d5767a
Reviewed-by: Wouter Huysentruit <wouter_huysentruit@hotmail.com>
Reviewed-by: Christian Stromme <christian.stromme@digia.com>
Implement customFocusPoint in camerabin backend.

Send a regions-of-interest event containing a rectangle surrounding the
focus point to the camera source when the camera's focusPointMode
is QCameraFocus::CustomFocusPoint.  Reset the regions-of-interest if the
focusPointMode changes to any other value.

[ChangeLog][QtMultimedia][GStreamer] Implement customFocusPoint in camerabin backend.

Change-Id: If75405733b18d476dff5467abae3f08271caf06a
Add a control which allows overriding the gstreamer video sink.

This enables applications to selectively use platform specific or
unusual (video4linux perhaps) gstreamer sink elements to render video.

Change-Id: Ife2c2f9b2149d6b598c69a78f1e116ba05b46e10
MER: Add a sensor control which returns orientation information.

Change-Id: I7542cc54a253031b0692780f368cf31c65182d96
MER: Build against Qt 5.1

Change-Id: Ib22e45b6be2d8493f30fe664c6ee09d22b407205