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).

tests/ceftests/run_all_unittests.cc

@@ -18,6 +18,7 @@
 #endif
 
 #include "include/base/cef_callback.h"
+#include "include/cef_api_hash.h"
 #include "include/cef_app.h"
 #include "include/cef_task.h"
 #include "include/cef_thread.h"
@@ -136,6 +137,10 @@ class ScopedPlatformSetup final {
 int main(int argc, char* argv[]) {
   int exit_code;
 
+#if CEF_API_VERSION != CEF_EXPERIMENTAL
+  printf("Running with configured CEF API version %d\n", CEF_API_VERSION);
+#endif
+
 #if defined(OS_WIN) && defined(ARCH_CPU_32_BITS)
   // Run the main thread on 32-bit Windows using a fiber with the preferred 4MiB
   // stack size. This function must be called at the top of the executable entry