Include proper license text (GPLv3)

Currently, each Python file (`controller.py` and `gui.py`) states that it is
licensed under version 3 of the GPL, whereas the included license copy,
(`LICENSE.txt`) is actually version 2.

This commit removes the outdated license and includes the proper text
for GPLv3.

Note: The license file is named `COPYING`, per the FSF's
recommendation.

ref https://www.gnu.org/licenses/gpl-howto.html
fix asylumfunk/1