Add a method to clear launch screen cache

[fredpi]

I propose to add the following extension to UIApplication, taken from this article:

public extension UIApplication {
    func clearLaunchScreenCache() {
        do {
            try FileManager.default.removeItem(atPath: NSHomeDirectory()+"/Library/SplashBoard")
        } catch {
            print("Failed to delete launch screen cache: \(error)")
        }
    }
}

While this extension is only appropriate in debug environments, I think it's still a good fit for HandyUIKit...