NPanel refactor - 1st pass: SidePanel and settings an new logic

Services/PanelService.qml

@@ -5,8 +5,16 @@ import Quickshell
 Singleton {
   id: root
 
+  // A ref. to the sidePanel, so it's accessible from other services
+  property var sidePanel: null
+
   // Currently opened panel
   property var openedPanel: null
 
-  property var sidePanel: null
+  function registerOpen(panel) {
+    if (openedPanel && openedPanel != panel) {
+      openedPanel.close()
+    }
+    openedPanel = panel
+  }
 }