FolderNodes: Specify filter for Add Existing Directory

Allow foldernodes to specify which files they care for when doing
"Add Existing Directory". Before we used a fixed list with C++ header
and source files, which does make no sense for e.g. adding to resources.

Task-number: QTCREATORBUG-15278
Change-Id: I15dad133391485c2bcebd2d864623304b31b5f8f
Reviewed-by: Niels Weber <niels.weber@theqtcompany.com>

src/plugins/resourceeditor/resourcenode.cpp

@@ -201,6 +201,11 @@ void ResourceTopLevelNode::update()
     }
 }
 
+QString ResourceTopLevelNode::addFileFilter() const
+{
+    return QLatin1String("*.png; *.jpg; *.gif; *.svg; *.ico; *.qml; *.qml.ui");
+}
+
 QList<ProjectExplorer::ProjectAction> ResourceTopLevelNode::supportedActions(ProjectExplorer::Node *node) const
 {
     if (node != this)