Feature Request: Specify screen for Scratchpads

[gennarocc]

I would like the ability to specify a screen in the scratchpad props.

In my set up I use a vertical monitor and for simplicity I would only like to display the scratchpads on my main horizontal one.

Trying alternate solutions such as the following don't seem to work.

music_control:connect_signal("turn_on", function(c) c.screen = screen.primary end)