Fix build palette showing too many queued items when BuildLimit is reached

When a BuildLimit on an actor type is set and nearly reached, you could
shift-click the build palette and it would show it had five more items
queued when really less are allowed to be built. This fixes it so that
only the allowed number of items is enqueued, and thus showing the
correct number on the build palette.
Merge pull request #5188 from obrakmann/buildlimit+shift-click-fix

Fix build palette showing too many queued items when BuildLimit is reached