Add a "License:" tag to all code files.

In cases where there is no license specified, the file is tagged as
"License: Unspecified".  Obviously, that is not ideal, but it
highlights the fact, and I hope that it encourages whoever has the
authority to specify the license to do so.

On that note, to anyone who may have the authority to specify the
license of files in devtools: the current licence of many files is
GPLv2 with no option for later versions; I impore you to re-license
them to have the "or any later version" option.