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/pythoneditor/pythoneditorplugin.cpp

@@ -337,6 +337,8 @@ public:
 
     QList<ProjectAction> supportedActions(Node *node) const override;
 
+    QString addFileFilter() const override;
+
     bool renameFile(const QString &filePath, const QString &newFilePath) override;
     void refresh(QSet<QString> oldFileList = QSet<QString>());
 
@@ -1057,6 +1059,11 @@ QList<ProjectAction> PythonProjectNode::supportedActions(Node *node) const
     return {};
 }
 
+QString PythonProjectNode::addFileFilter() const
+{
+    return QLatin1String("*.py");
+}
+
 bool PythonProjectNode::renameFile(const QString &filePath, const QString &newFilePath)
 {
     return m_project->renameFile(filePath, newFilePath);