Add a parameter in the DisplayCueImage box to not send the triggers to the acquisition server.

It is sometimes useful to create a control display that shows the cued images to the experimenter (not the subject) on a different screen. But doing so, duplicated events (one per display) are sent to the acquisition server. This is wrong. This PR adds a boolean parameter to the DisplayCueImage box. By default this parameter is true (i.e. triggers are sent), but using this parameter it is possible to add multiple DisplayCueImage boxes but have only one that sends the triggers to the acquisition server.

The parameter is set to true by default to keep the old behaviour. However, this setup can lead to scenarios in which multiple triggers are sent, without the experimenter noticing it immediately. Putting the parameter to false by default would have the advantage that the experimenter would immediately see that sthg is wrong (triggers would not appear on the signal display)...

Note also that the testing has been done only at the interface level as lua boxes currently do not work on linux (see issue #3 (closed)).