- Introduce native/ and osr/ folders for native (non-platform-agnostic) and
    osr (windowless) code respectively.
  - Introduce CefBrowserPlatformDelegate for abstracting platform-specific
    implementations of browser host functionality.
  - Move dialog and menu code to separate manager and platform-specific runner
    implementations exposed via CefBrowserPlatformDelegate.
  - Standardize focus-handling behavior between windowed and windowless
    implementations. CefFocusHandler::OnSetFocus() will now also be called for
    osr focus changes.
- Support multiple simultaneous popups (issue #1289).