Part: QFileDialog asks to overwrite a file on its own

• No need to stat the url we get from QFileDialog::getSaveFileUrl, if it already exists the dialog should show the "do you want to overwrite?" message to the user on its own; also no need to check for isValid, the dialog will return valid urls, AFAICS
• KIO::file_copy will check if the src url exists ...etc, we don't have to do that manually; also make the job async, and re-use the custom error messages where possible