Indicate coming/missing soldiers in soldier displays

[Benedikt Straub]

Like for wares (see bug 1781732), I´d like to be able to see in the soldier displays in military- and trainingsites whether a soldier is already coming to a vacant slot or if there aren´t enough soldiers in the economy.

The question is whether it should only be indicated _that_ a soldier is coming, or what _levels_ and _health_ the coming soldier has, since this can have an influence on tactics. I vote for the latter.

[GunChleoc (gunchleoc)]

To avoid merge conflicts, we should merge https://code.launchpad.net/~widelands-dev/widelands/bug-794407-soldier-stats before working on this.

[GunChleoc (gunchleoc)]

Migrated to https://github.com/widelands/widelands/issues/2122