Since everything uses the same warning number, the old setup resulted in [[deprecated]] being silenced.
So this explicitly adds the needed #defines to silence the very noisy warning from the MSVC headers but leaves warning 4996 active otherwise.
In particlular this does:
* silence all warnings in the subprojects
* do not derive TIterator from std::iterator anymore as C++17 deprecates this.
* silence the above for RapidJSON because altering that code is not desirable.
* explicitly disable warning 4996 in some Windows files that call the deprecated (but still needed) GetVersionEx function.
* define _CRT_SECURE_NO_DEPRECATE, _CRT_SECURE_NO_WARNINGS and _CRT_NONSTDC_NO_WARNINGS through CMake to disable the CRT's deprecation and security warnings.
Currently this will print several (intended) deprecation warnings about 'updatesector' that point to code that needs to be changed but cannot yet without other refactorings being done first.
Renamed all elements still referring to zdoom.
removed the frontend specific resource data.
fixed startup dialog to accept ANSI date despite building as Unicode. This needed a bit of hackery because the macros in windowsx.h are not character set sensitive.
Currently none of these is being used, but eventually they will, once more code gets ported over.
So it's better to have them right away and avoid editing the project file too much, only to revert that later.
