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_command_handler.h

@@ -50,12 +50,13 @@ class CefCommandHandler : public virtual CefBaseRefCounted {
  public:
   ///
   /// Called to execute a Chrome command triggered via menu selection or
-  /// keyboard shortcut. Values for |command_id| can be found in the
-  /// cef_command_ids.h file. |disposition| provides information about the
-  /// intended command target. Return true if the command was handled or false
-  /// for the default implementation. For context menu commands this will be
-  /// called after CefContextMenuHandler::OnContextMenuCommand. Only used with
-  /// Chrome style.
+  /// keyboard shortcut. Use the cef_id_for_command_id_name()
+  /// function for version-safe mapping of command IDC names from
+  /// cef_command_ids.h to version-specific numerical |command_id| values.
+  /// |disposition| provides information about the intended command target.
+  /// Return true if the command was handled or false for the default
+  /// implementation. For context menu commands this will be called after
+  /// CefContextMenuHandler::OnContextMenuCommand. Only used with Chrome style.
   ///
   /*--cef()--*/
   virtual bool OnChromeCommand(CefRefPtr<CefBrowser> browser,
@@ -65,9 +66,11 @@ class CefCommandHandler : public virtual CefBaseRefCounted {
   }
 
   ///
-  /// Called to check if a Chrome app menu item should be visible. Values for
-  /// |command_id| can be found in the cef_command_ids.h file. Only called for
-  /// menu items that would be visible by default. Only used with Chrome style.
+  /// Called to check if a Chrome app menu item should be visible. Use the
+  /// cef_id_for_command_id_name() function for version-safe mapping of command
+  /// IDC names from cef_command_ids.h to version-specific numerical
+  /// |command_id| values. Only called for menu items that would be visible by
+  /// default. Only used with Chrome style.
   ///
   /*--cef()--*/
   virtual bool IsChromeAppMenuItemVisible(CefRefPtr<CefBrowser> browser,
@@ -76,9 +79,11 @@ class CefCommandHandler : public virtual CefBaseRefCounted {
   }
 
   ///
-  /// Called to check if a Chrome app menu item should be enabled. Values for
-  /// |command_id| can be found in the cef_command_ids.h file. Only called for
-  /// menu items that would be enabled by default. Only used with Chrome style.
+  /// Called to check if a Chrome app menu item should be enabled. Use the
+  /// cef_id_for_command_id_name() function for version-safe mapping of command
+  /// IDC names from cef_command_ids.h to version-specific numerical
+  /// |command_id| values. Only called for menu items that would be enabled by
+  /// default. Only used with Chrome style.
   ///
   /*--cef()--*/
   virtual bool IsChromeAppMenuItemEnabled(CefRefPtr<CefBrowser> browser,