Use SPDX for licenses (#2122)

* Use SPDX identifier in header files

* Use SPDX identifier in rest of source files

* Fix usage of SPDX for files with 2 licenses

* Add global license file

* Fix compilation

unitTests/test_bmpimage.cpp

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+
 #include <gtest/gtest.h>
 
 #include <array>