Include SPDX short license identifiers into all WZ's own source files

[ManManson]

Include a GPL-2.0-or-later SPDX licence identifier at the top of each WZ's source file. All other meta-info in file headers (copyright statements, licence notices, etc.) should be retained.

Note, that the project contains 3rdparty submodules and some other invidivial files, which are licensed conditions conditions different from the Warzone 2100 itself. These files are not subject to this particular proposal.

For the list of why it is beneficial to use short SPDX identifiers, please refer to https://spdx.dev/learn/handling-license-info/#why