new trigger function for setting dynamite/satchel radius

[ETLegacyTeam]

Author: @realkemon (keMoN)
Date: 2014-02-23
Redmine Issue: 524, https://dev.etlegacy.com/issues/524

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In vanilla ET there is a hardcoded radius in which the player is able to plant dynamite/satchel. This results in various trickplant options, which might not be desired/intended by the mapper.
(eg. goldrush 2nd tankbarrier dynamite on stairs; oasis guns outside)

Introducing a new trigger function (maybe trigger_constructible) would be handy for mappers. That trigger would specify the area in which an objective is affected by dynamite/satchel.
If I'm correct, "this":http://games.chruker.dk/enemy_territory/entities.php?action=show_entity&entity_name=func_constructible is the entity which is normally the center of the radius.