Add initial support for API versioning (see #3836)

- Generated files are now created when running cef_create_projects or
  the new version_manager.py tool. These files are still created in the
  cef/ source tree (same location as before) but Git ignores them due to
  the generated .gitignore file.
- API hashes are committed to Git as a new cef_api_versions.json file.
  This file is used for both code generation and CEF version calculation
  (replacing the previous usage of cef_api_hash.h for this purpose).
  It will be updated by the CEF admin before merging breaking API
  changes upstream.
- As an added benefit to the above, contributor PRs will no longer
  contain generated code that is susceptible to frequent merge conflicts.
- From a code generation perspective, the main difference is that we now
  use versioned structs (e.g. cef_browser_0_t instead of cef_browser_t)
  on the libcef (dll/framework) side. Most of the make_*.py tool changes
  are related to supporting this.
- From the client perspective, you can now define CEF_API_VERSION in the
  project configuration (or get CEF_EXPERIMENTAL by default). This
  define will change the API exposed in CEF’s include/ and include/capi
  header files. All client-side targets including libcef_dll_wrapper
  will need be recompiled when changing this define.
- Examples of the new API-related define usage are provided in
  cef_api_version_test.h, api_version_test_impl.cc and
  api_version_unittest.cc.

To test:
- Run `ceftests --gtest_filter=ApiVersionTest.*`
- Add `cef_api_version=13300` to GN_DEFINES. Re-run configure, build and
  ceftests steps.
- Repeat with 13301, 13302, 13303 (all supported test versions).

cef_paths2.gypi

@@ -36,7 +36,6 @@
       'include/base/internal/cef_thread_checker_impl.h',
       'include/cef_api_hash.h',
       'include/cef_base.h',
-      'include/cef_version.h',
       'include/internal/cef_export.h',
       'include/internal/cef_ptr.h',
       'include/internal/cef_string_wrappers.h',
@@ -44,6 +43,8 @@
       'include/internal/cef_types_wrappers.h',
     ],
     'includes_common_capi': [
+      'include/cef_id_mappers.h',
+      'include/cef_version_info.h',
       'include/internal/cef_dump_without_crashing_internal.h',
       'include/internal/cef_logging_internal.h',
       'include/internal/cef_string.h',
@@ -483,6 +484,7 @@
       'tests/ceftests/resources/net/data/ssl/certificates/root_ca_cert.pem',
     ],
     'ceftests_sources_common': [
+      'tests/ceftests/api_version_unittest.cc',
       'tests/ceftests/audio_output_unittest.cc',
       'tests/ceftests/browser_info_map_unittest.cc',
       'tests/ceftests/certificate_error_unittest.cc',