Add SignalTrigger

[hgmarques]

Regarding the discussion online. I believe the addition of the new property SignalTrigger is rather harmless when it comes to potential restrictions on future reorganization of this package. This is because the SignalTrigger is kind of analogous to the SignalSource property which is already in place; and so, future changes in the package would also require changes in the already existing SignalSource property.

[CLAassistant]

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:white_check_mark: hgmarques
:x: maduarte95
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