Sound event trigger for sound effects

[Pureon]

This is to use in cases where sound effects should start either before or after the 'event' trigger.

For siege weapons and probably other units we need a new actor time value that can be set separately from the 'event' trigger which fires when the weapon's projectile is released. Our attack sound effects currently start on the 'event', but for catapults and some other units the sound effects should start much earlier - for example when the Roman Ballista is being cranked ready to fire, that's when the cranking sound should start.

What we have now is:

<animation event="0.75" load="0.7" file="mechanical/rome_ballista_atttack.dae" name="Melee" speed="150"/>

Which fires the sound and projectile at event="0.75" and the projectile appears on the Ballista at load="0.7"

What we would like to have is something like this:

<animation event="0.75" load="0.7" sound="0.1" file="mechanical/rome_ballista_atttack.dae" name="Melee" speed="150"/>

Here the sound would start to play at sound="0.1" of the catapult cranking + firing animation.

Where there is no 'sound' trigger, the usual 'event' trigger will be used to start the sound effect.

[historic_bruno]

Duplicate of #1944.