LayoutSaver now can save/restore a sub-set of windows

Each subset is identified by its "affinity name".
See DockWidget/MainWindow setAffinityName property.

Patch contributed by Andras Mantia, I've changed it to deal in
terms of affinity names instead of unique names.

src/private/multisplitter/MultiSplitterLayout.cpp

@@ -604,6 +604,19 @@ void MultiSplitterLayout::ensureItemsMinSize()
     }
 }
 
+QString MultiSplitterLayout::affinityName() const
+{
+    if (auto ms = multiSplitter()) {
+        if (auto mainWindow = ms->mainWindow()) {
+            return mainWindow->affinityName();
+        } else if (auto fw = ms->floatingWindow()) {
+            return fw->affinityName();
+        }
+    }
+
+    return QString();
+}
+
 void MultiSplitterLayout::addMultiSplitter(MultiSplitter *sourceMultiSplitter,
                                            Location location,
                                            Frame *relativeTo)