notifications: add modal IPC command for dismissing all popups. rename clearAllPopups() to dismissAllPopups(), since clear is otherwise used to mean eliminated entirely rather than just sent to the notification center. (#1100)

quickshell/Services/NotificationService.qml

@@ -403,7 +403,7 @@ Singleton {
         NotifWrapper {}
     }
 
-    function clearAllPopups() {
+    function dismissAllPopups() {
         for (const w of visibleNotifications) {
             if (w) {
                 w.popup = false;