UnitMotion - Send messages to UnitAI when obstructed, to allow stopping early when walking and avoiding pathfinding lag.

As reported by #5521, Ordering units to walk to a point in a forest can lag terribly, as units will end up computing long short paths in the forest, which can result in ComputeShortPath calls that individually take upwards of 80ms.

This can be alleviated by allowing units to stop a bit earlier. A23 handled this in UnitMotion directly, but it's better to handle this in UnitAI to avoid surprises and to make it customisable on a per-state level.

This diff further speeds up using the short pathfinder by starting with a smaller search range, limiting the max-range more and moving the range slightly towards the goal.

This also refactors UM sending messages to UnitAI so that we may in the future push more information (in particular, the entity_id that a unit was obstructed by could be interesting for COMBAT).

This doesn't fix the possibility of lag, but it reduces its occurrence to levels that should be similar to A23 and thus acceptable enough.

Tested By: Freagarach

Fixes #5521

Differential Revision: https://code.wildfiregames.com/D2105