Announce user's decision about DND (#870)

This will be used in `pcmanfm-qt` to fix a small regression that was caused by showing DND menu after finishing DND (for Wayland).

src/folderview.cpp

@@ -1655,6 +1655,8 @@ void FolderView::childDropEvent(QDropEvent* e) {
                     break;
                 }
 
+                Q_EMIT dropIsDecided(action != Qt::IgnoreAction);
+
                 switch(action) {
                 case Qt::CopyAction:
                     FileOperation::copyFiles(srcPaths, destPath);
@@ -1670,8 +1672,13 @@ void FolderView::childDropEvent(QDropEvent* e) {
                 }
             });
             e->accept(); // prevent further event propagation
+            return;
         }
     }
+
+    QTimer::singleShot(0, view, [this] {
+        Q_EMIT dropIsDecided(true); // after finishing drop
+    });
 }
 
 bool FolderView::eventFilter(QObject* watched, QEvent* event) {

src/folderview.h

@@ -192,6 +192,8 @@ private Q_SLOTS:
 
     void inlineRenamed(const QString& oldName, const QString& newName);
 
+    void dropIsDecided(bool accepted);
+
 private:
 
     QAbstractItemView* view;