Normalize license headers of editor/benchmark/tests/firefox-addon/viewer source files

* remove @license tag, for consistency with library source files
* use copyright@kogmbh.com email address everywhere
* remove "additional permission", to be added only to released stuff
* separate license header by empty line from following content

NOT cared for, to avoid conflicts with android work of @vandenoever:
* programs/android
* programs/touchui

Open questions:
* firefox extension is listed as GPL 3.0 on the [Add-on page](https://addons.mozilla.org/de/firefox/addon/webodf/), not AGPL 3.0. Should perhaps the add-on code also be "just" GPL 3.0?
* Should the programs and the FF add-on get a different text? "This file is part of WebODF. WebODF is free software [...]" might not match perfectly, unless "WebODF" means the project here, not the library. Even more interesting for ViewerJS source files in programs/viewer