### Preflight Checklist
* [x] I have read the [Contributing Guidelines](https://github.com/electron/electron/blob/master/CONTRIBUTING.md) for this project.
* [x] I agree to follow the [Code of Conduct](https://github.com/electron/electron/blob/master/CODE_OF_CONDUCT.md) that this project adheres to.
* [x] I have searched the issue tracker for a feature request that matches the one I want to file, without success.

### Problem Description
When working with BrowserViews, there's no way to listen to focus changes on either BrowserView itself or its WebContents.

### Proposed Solution
On BrowserWindow, there are `focus` and `blur` events. The same events need to be emitted on BrowserView/WebContents.
i.e. : 
```
browserView.on('focus', () => {
    // view is focused, do stuff
});
```
or  
```
browserView.webContents.on('focus', () => {
    // contents are focused, do stuff
});
```

### Alternatives Considered
I can't think of a good way of working around this lack of functionality
shell/browser/api/electron_api_web_contents.cc
std::string accept_languages =
      g_browser_process->GetApplicationLocale() + ",";
  for (auto const& language : electron::GetPreferredLanguages()) {
    if (language == g_browser_process->GetApplicationLocale())
      continue;
    accept_languages += language + ",";
  }
  accept_languages.pop_back();
  prefs->accept_languages = accept_languages;
#if defined(OS_LINUX) || defined(OS_WIN)
  
  static const gfx::FontRenderParams params(
      gfx::GetFontRenderParams(gfx::FontRenderParamsQuery(), nullptr));
  prefs->should_antialias_text = params.antialiasing;
  prefs->use_subpixel_positioning = params.subpixel_positioning;
  prefs->hinting = params.hinting;
  prefs->use_autohinter = params.autohinter;
  prefs->use_bitmaps = params.use_bitmaps;
  prefs->subpixel_rendering = params.subpixel_rendering;
#endif
  
  if (auto interval = GetCursorBlinkInterval())
    prefs->caret_blink_interval = *interval;
  
  
  
  if (!WebContentsPreferences::From(ontents())) {
    new WebContentsPreferences(ontents(), options);
  }