Add GPLv2 COPYING file to repository and indicate license in each file

[ryandesign]

The README says:

The microM8 GUI is GPLv2 licensed.

but this is the only place where the words "license" or "GPL" appear in this repository which is not sufficient. Please copy the GPLv2 COPYING file into this repository and indicate at the top of each file that it is released under that license. Read https://www.gnu.org/licenses/gpl-faq.en.html#WhyMustIInclude for why.