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).

include/cef_resource_bundle_handler.h

@@ -53,8 +53,10 @@ class CefResourceBundleHandler : public virtual CefBaseRefCounted {
   ///
   /// Called to retrieve a localized translation for the specified |string_id|.
   /// To provide the translation set |string| to the translation string and
-  /// return true. To use the default translation return false. Include
-  /// cef_pack_strings.h for a listing of valid string ID values.
+  /// return true. To use the default translation return false. Use the
+  /// cef_id_for_pack_string_name() function for version-safe mapping of string
+  /// IDS names from cef_pack_strings.h to version-specific numerical
+  /// |string_id| values.
   ///
   /*--cef()--*/
   virtual bool GetLocalizedString(int string_id, CefString& string) = 0;
@@ -64,8 +66,9 @@ class CefResourceBundleHandler : public virtual CefBaseRefCounted {
   /// To provide the resource data set |data| and |data_size| to the data
   /// pointer and size respectively and return true. To use the default resource
   /// data return false. The resource data will not be copied and must remain
-  /// resident in memory. Include cef_pack_resources.h for a listing of valid
-  /// resource ID values.
+  /// resident in memory. Use the cef_id_for_pack_resource_name() function for
+  /// version-safe mapping of resource IDR names from cef_pack_resources.h to
+  /// version-specific numerical |resource_id| values.
   ///
   /*--cef()--*/
   virtual bool GetDataResource(int resource_id,
@@ -77,8 +80,10 @@ class CefResourceBundleHandler : public virtual CefBaseRefCounted {
   /// factor |scale_factor|. To provide the resource data set |data| and
   /// |data_size| to the data pointer and size respectively and return true. To
   /// use the default resource data return false. The resource data will not be
-  /// copied and must remain resident in memory. Include cef_pack_resources.h
-  /// for a listing of valid resource ID values.
+  /// copied and must remain resident in memory. Use the
+  /// cef_id_for_pack_resource_name() function for version-safe mapping of
+  /// resource IDR names from cef_pack_resources.h to version-specific numerical
+  /// |resource_id| values.
   ///
   /*--cef()--*/
   virtual bool GetDataResourceForScale(int resource_id,