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