[iOS] Refactoring enhancement ideas

- [ ] Move view controller logic in Manager into a KMW view controller
- [ ] Remove setting the URL hash in KeymanWeb (since UIWebView is not longer used) and post an Object rather than String into WebKit
- [ ] Use promises/callbacks for HTTP rather than storing state in HTTPDownloadRequest and HTTPDownloader
- [ ] Remove code for background gradients in some views if unnecessary (eg. PopoverView, KeyPreviewView, SubKeysView)
- [ ] Refactor duplicated drawing code in the above views
- [ ] KeyboardMenuList list of keyboards should not contain the close button
- [ ] Reorganize groups in Xcode projects
- [x] Logging should be moved out of Manager. Also consider using a logging library and more log levels.
- [ ] Potentially use delegates instead of NotificationCenter