Restore copyright headers in split preferences files

OK, so every source file needs a license header, and every license header needs at least one copyright line. I missed this in code review. For the source files split from ephy-prefs.c, we need to preserve the original copyright lines from that file:

 *  Copyright © 2000-2003 Marco Pesenti Gritti
 *  Copyright © 2003, 2004, 2005 Christian Persch
 *  Copyright © 2010, 2017 Igalia S.L.

Note that we don't need to preserve the typo (the original file says © 200-2003, which is obviously wrong). You can add yourself on a fourth line if you want.

For the headers, I would just add yourself:

 * Copyright © 2020 Andrei Lisita

It's good to get your name into source files, but if for some reason you don't want that, then we can use a generic copyright line like Copyright © 2020 Epiphany Developers. We just have to have something there, since you can't have a copyright license without a copyright line.

CC @Yetizone