Warn user if a paste operation is in progress

src/timeline2/model/timelinefunctions.cpp

@@ -1620,6 +1620,9 @@ bool TimelineFunctions::pasteClips(const std::shared_ptr<TimelineItemModel> &tim
                                    Fun &redo)
 {
     timeline->requestClearSelection();
+    if (!semaphore.tryAcquire(1)) {
+        pCore->displayMessage(i18n("Another paste operation is in progress"), ErrorMessage, 500);
+    }
     while (!semaphore.tryAcquire(1)) {
         qApp->processEvents();
     }