Add SPDX license (but not copyright) headers to all files which follow a
certain pattern in their existing non-machine-readable header comment.
This commit was entirely generated using the command:
```
git ls-files gobject/tests/*.[ch] | xargs perl -0777 -pi -e 's/\n \*\n \* This library is free software; you can redistribute it and\/or\n \* modify it under the terms of the GNU Lesser General Public/\n \*\n \* SPDX-License-Identifier: LGPL-2.1-or-later\n \*\n \* This library is free software; you can redistribute it and\/or\n \* modify it under the terms of the GNU Lesser General Public/igs'
```
Signed-off-by: Philip Withnall <pwithnall@endlessos.org>
Helps: #1415
Originally, GBindingGroup started with Builder as a way to simplify all
of the third-degree object bindings necessary around Model-Controller
objects such as TextBuffer/TextView.
Over time, it has grown to be useful in a number of scenarios outside
of Builder and has been copied into a number of projects such as GNOME
Text Editor, GtkSourceView, libdazzle, and more.
It makes sense at this point to unify on a single implementation and
include that upstream in GObject directly alongside GBinding.
