If the statusline generator (i.e. i3status) specifies click_events:true
in the protocol header, i3bar will write a JSON array on it's stdin
notifying it if the user clicks on a block.
The exact protocol is documented in docs/i3bar-protocol.
We now wait for the child process to send the first line before stopping
it to use the signal which might be specified in the i3bar protocol
header
Since clients might use the same signal for both stop and cont, we also
save the stopped state of the child to avoid stopping it while hidden!
