Make checksum check option more visible and friendly

aria2c provides checksum checking via the option `--checksum=<TYPE>=<DIGEST>` but this option is hidden waaaay down the list of *advanced* options. It also requires the user to input it in the form of `<TYPE>=<DIGEST>`. It would be much better if this were included in the basic download options rather than advanced, and the permissible digest algorithms (sha-1 etc) were provided in a combobox so the user can just copy-paste the digest itself without worrying about the spelling of the algorithm etc.