QmlDesigner: Add drag support to empty asset library placeholder item

Fixes: QDS-6430
Change-Id: I561f4ed51ccb1e47472adb9baa6e5594eb93eafb
Reviewed-by: Mahmoud Badri <mahmoud.badri@qt.io>

share/qtcreator/qmldesigner/itemLibraryQmlSources/Assets.qml

@@ -469,6 +469,17 @@ Item {
             visible: assetsModel.isEmpty && searchBox.isEmpty()
             clip: true
 
+            DropArea { // handles external drop (goes into default folder based on suffix)
+                anchors.fill: parent
+
+                onEntered: (drag)=> {
+                    root.updateDropExtFiles(drag)
+                }
+
+                onDropped: {
+                    rootView.handleExtFilesDrop(root.dropSimpleExtFiles, root.dropComplexExtFiles)
+                }
+
                 Column {
                     id: colNoAssets
 
@@ -515,6 +526,7 @@ Item {
                     }
                 }
             }
+        }
 
         ScrollView { // TODO: experiment using ListView instead of ScrollView + Column
             id: assetsView

src/plugins/qmldesigner/components/assetslibrary/assetslibrarywidget.cpp

@@ -218,13 +218,18 @@ void AssetsLibraryWidget::handleExtFilesDrop(const QStringList &simpleFilesPaths
                                              const QString &targetDirPath)
 {
     if (!simpleFilesPaths.isEmpty()) {
-        AddFilesResult result = ModelNodeOperations::addFilesToProject(simpleFilesPaths, targetDirPath);
+        if (targetDirPath.isEmpty()) {
+            addResources(simpleFilesPaths);
+        } else {
+            AddFilesResult result = ModelNodeOperations::addFilesToProject(simpleFilesPaths,
+                                                                           targetDirPath);
             if (result == AddFilesResult::Failed) {
                 Core::AsynchronousMessageBox::warning(tr("Failed to Add Files"),
                                                       tr("Could not add %1 to project.")
                                                           .arg(simpleFilesPaths.join(' ')));
             }
         }
+    }
 
     if (!complexFilesPaths.empty())
         addResources(complexFilesPaths);

src/plugins/qmldesigner/components/assetslibrary/assetslibrarywidget.h

@@ -82,7 +82,7 @@ public:
     Q_INVOKABLE void handleSearchfilterChanged(const QString &filterText);
     Q_INVOKABLE void handleExtFilesDrop(const QStringList &simpleFilesPaths,
                                         const QStringList &complexFilesPaths,
-                                        const QString &targetDirPath);
+                                        const QString &targetDirPath = {});
     Q_INVOKABLE QSet<QString> supportedAssetSuffixes(bool complex);
 
 signals: