Code review comment for lp://staging/~widelands-dev/widelands/bug-1695701-militarywindow-crash

The problem was that the old building gets destroyed and a new one created in the same spot. This is partially done via scheduled playercommands, so we can have nondeterministic delays here.

Sending a note to the building window fixes the issue.

I'd still like to look some more at that conquering code though - it's overly complicated. Before the OneTribe change, we used to have "global" militarysites so that we could add other tribes' building types when conquering. This is no longer needed, so I think rather than destroying and recreating, we should be able to simply unconquer for old owner, change owner and conquer for new owner.