Make project compatible with Reuse Software spec

Provide all licenses used in the project in a LICENSES folder and
add SPDX license and copyright information for all files in
accordance with the Reuse Software[0] specification.

The copyright information is based on the file's git history,
using a fairly generous definition of "non-trivial".

As of the spec recommendation, the information is generally added
as comments in the files themselves, except for

 - NEWS, README and similar top-level standard files, so that
   a SPDX code isn't the first thing people encounter
 - files that don't support comments (json) or where they'd
   be a bit awkward (.desktop, .service)
 - anything under po/, to not interfere with translation teams

Those are covered by a .reuse/dep5 files, except for image assets,
where separate .license files are used (It would be possible to
add comments to SVG files, but I don't trust image editors to
preserve them).

[0] https://reuse.software/

Part-of:
<https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/224>

.gitlab-ci/commit-rules.yml

@@ -1,3 +1,7 @@
+# SPDX-FileCopyrightText: 2021 Florian Müllner <fmuellner@gnome.org>
+#
+# SPDX-License-Identifier: GPL-2.0-or-later
+
 patterns:
   deny:
     - regex: '^$CI_MERGE_REQUEST_PROJECT_URL/(-/)?merge_requests/$CI_MERGE_REQUEST_IID$'