Add the GPLv3 license text

LGPLv3 refers to it but does not include it in its body.

Change-Id: Ib056b47dde3341ef9a52ffff13eed18cf3504738
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>

LICENSE.LGPLv3

+            GNU LESSER GENERAL PUBLIC LICENSE
+
+ The Qt Toolkit is Copyright (C) 2015 The Qt Company Ltd.
+ Contact: http://www.qt.io/licensing/
+
+ You may use, distribute and copy the Qt GUI Toolkit under the terms of
+ GNU Lesser General Public License version 3, which is displayed below.
+ This license makes reference to the version 3 of the GNU General
+ Public License, which you can find in the LICENSE.GPLv3 file.
+
+-------------------------------------------------------------------------
+
                    GNU LESSER GENERAL PUBLIC LICENSE
                        Version 3, 29 June 2007