This commit requires D274 and D281.

I have been porting over the classic "Roll The Dice" scenario from Age of Empires to 0 A.D.
It's a very fun scenario, checkout out ZeroEmpires' video about it (e.g. https://www.youtube.com/watch?v=jAW_u62H1Vk).

This map also showcases the extreme flexibility of our trigger scripts, by changing simulation code without mods and even adding a custom ability to make the "abilities" such as the panic-button integrated into the GUI instead of using a workaround, like in Age of Empires.

I have not reimplemented everything, and this does requires 2 changes:
-Triggers support the "Attacked" message
-All units have an empty Abilities component by default, to support adding custom abilities.

Start a Roll The Dice scenario. Setting some players as AI will remove them, so leave them unassigned. This was done to have a scenario that still allows variable number of players, but it's a little broken obviously.