add a "force recompute pathing" button to the stranded unit dialog

[ab9rf]

DF currently has a bug that often causes the walkability map to be incorrectly updated when a building is constructed, resulting in the walkabilty map being incorrect and leading to stranded units. Setting the reindex_pathfinding flag forces DF to recompute the entire walkability map and resolves issues of this sort. Adding a button to the "stranded unit" dialog of warn-stranded would give players a quick recovery for whenever this bug impacts their gameplay.

Of course, :lua world.reindex_pathfinding = 1 can always be used at any time to force a recompute.

[quietust]

See also #5511