Fortify - Allow custom deploy handlers

[Cuel]

When merged this pull request will:

• Allow mission makers to register deployment handlers/validators so that they get more control of when a placement is possible. Inspired by ace explosives handlers.
• Adds a local event when deployment starts, e.g for displaying hints or possibilities of placement

Use case examples:

• Mission maker can control max height of placement
• Mission maker can check for nearby objects / disallow placing things too close

[Tennessee-Timmy]

Whoa, I was just looking for how to do this.
I need to have only a certain number of certain items to be allowed to use to fortify.

Would be nice to do this. Would really open up some doors.