Make it obvious what platforms are supported by QtSensors.

Also, this brings the front page layout somewhat further
in line with qtlocation & qtconnectivity API docs.

Change-Id: I5e54ae388892dcb2a806c4775828abcdc81eb79d
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Merge remote-tracking branch 'origin/stable' into dev

Change-Id: I6b203a32b2f52255ae4e63cfb279e762458c0128