Add signature for all source files

[EndrII]

Signature example :

#
# Copyright (C) 2021-2021 QuasarApp.
# Distributed under the GPLv3 software license, see the accompanying
# Everyone is permitted to copy and distribute verbatim copies
# of this license document, but changing it is not allowed.
#

[EndrII]

Note

change please copyright of all snake file to GPLv3 and swap LICENSE.md file