ref: unify Split popup logic (#3848)

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

src/singletons/WindowManager.hpp

@@ -60,6 +60,10 @@ public:
     Window &createWindow(WindowType type, bool show = true,
                          QWidget *parent = nullptr);
 
+    // Use this method if you want to open a "new" channel in a popup. If you want to popup an
+    // existing Split or SplitContainer, consider using Split::popup() or SplitContainer::popup().
+    Window &openInPopup(ChannelPtr channel);
+
     void select(Split *split);
     void select(SplitContainer *container);