LayoutSaver: Fix rounding errors when doing a relative restore

After a restore with relative sizes the items might not be on a pixel
perfect position regarding to their anchors. Make sure they are after
the restore is complete.

src/private/multisplitter/MultiSplitterLayout_p.h

@@ -442,6 +442,7 @@ private:
     friend class Anchor;
     friend class TestDocks;
     friend class KDDockWidgets::Debug::DebugWindow;
+    friend class LayoutSaver;
 
     struct AnchorBounds {
         Anchor *side1;