Add license files

This clarifies redistribution conditions for contributors.

Comments in the source files refer to them. Some files are under GPL with
exceptions or LGPL with exceptions, some are BSD-licensed:

find -type f |xargs sed -n 's/.*\(\$QT_BEGIN_LICENSE:[^\$]*\$\).*/\1/p' |sort |uniq -c
    104 $QT_BEGIN_LICENSE:BSD$
    108 $QT_BEGIN_LICENSE:LGPL$

There's also one GFDL licensed file:
doc/global/qt-html-templates-offline.qdocconf

Change-Id: I924ca0649691a52e9d7641962350e6857d82f947
Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Reviewed-by: Pier Luigi Fiorini <pierluigi.fiorini@gmail.com>
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>