init: rename 'Trigger' to 'Event' and convert to std::variant

The term 'trigger' should be used in 'Action' to indicate what causes
the 'Action' to be executed.

The term 'event' should be used in ActionManager's queue to indicate
a state change that is checked against the 'triggers' of an 'Action' to
see if it should execute.

Convert the previous Trigger class to std::variant, as the latter is
better suited for this use.

Change-Id: I2558367c8318b536aa69fcec93793f1c12857ef5