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

@@ -5,6 +5,8 @@
 
  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.
 
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