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).

cmake/cef_variables.cmake.in

@@ -69,6 +69,20 @@ list(APPEND CEF_COMPILER_DEFINES
 option(USE_SANDBOX "Enable or disable use of the sandbox." ON)
 
 
+# Optionally configure the CEF API version by adding `-D api_version=XXXXX` to the
+# cmake command-line where XXXXX is the desired version number. For background see
+# https://bitbucket.org/chromiumembedded/cef/wiki/ApiVersioning.md
+if(DEFINED api_version)
+  string(LENGTH "${api_version}" length)
+  if (NOT length EQUAL 5 OR NOT api_version MATCHES "^[0-9]+$")
+    message(FATAL_ERROR "Expected a 5 digit number for api_version, got '${api_version}'")
+  endif()
+  list(APPEND CEF_COMPILER_DEFINES
+    CEF_API_VERSION=${api_version}
+  )
+endif()
+
+
 #
 # Linux configuration.
 #