Added x-attach-split-to-window command line arg (#2411)

Co-authored-by: pajlada <rasmus.karlsson@pajlada.com>

src/widgets/splits/SplitContainer.hpp

@@ -255,6 +255,9 @@ private:
     NotebookTab *tab_;
     std::vector<Split *> splits_;
 
+    std::unordered_map<Split *, pajlada::Signals::SignalHolder>
+        connectionsPerSplit_;
+
     bool isDragging_ = false;
 };