wip: different heights for jump arrows

Note that this is very incomplete, for two different reasons:
- it's O(n^2) or potentially worse
- it makes the GC work hard (it invokes `new` in
  `LCanvas.DragLayout.layout()`)

For the second reason this does not clearly fit the criteria of
CONTRIBUTING.md, but I have yet to figure out ways to work around this
(or make the algorithm more efficient).

core/src/mindustry/logic/LStatements.java

@@ -870,7 +870,7 @@ public class LStatements{
             table.table(this::rebuild);
 
             table.add().growX();
-            table.add(new JumpButton(() -> dest, s -> dest = s)).size(30).right().padLeft(-8);
+            table.add(new JumpButton(() -> dest, s -> dest = s, this.elem)).size(30).right().padLeft(-8);
 
             String name = name();