New messages: 'drop_to{,monitor}'

'send_to' was broken, '--next' and '--prev' can't be options, and I
don't want to restrain the name of the desktops with 'send_to
DESKTOP_NAME|next|prev [--follow]' or handle multiple options: 'send_to
(DESKTOP_NAME|(next|prev --relative)) [--follow]' so I just created new
messages.