Editor: move ownership of assist interface to processor

This way the base class can manage the lifetime of the interface object
and it doesn't need to be done in each implementation of perform.

Change-Id: Ie1ce742e31b688a337533ee6c57d376146e25ace
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Christian Stenger <christian.stenger@qt.io>
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/qmldesigner/components/bindingeditor/bindingeditorwidget.cpp

@@ -76,10 +76,10 @@ bool BindingEditorWidget::event(QEvent *event)
     return QmlJSEditor::QmlJSEditorWidget::event(event);
 }
 
-TextEditor::AssistInterface *BindingEditorWidget::createAssistInterface(
+std::unique_ptr<TextEditor::AssistInterface> BindingEditorWidget::createAssistInterface(
     [[maybe_unused]] TextEditor::AssistKind assistKind, TextEditor::AssistReason assistReason) const
 {
-    return new QmlJSEditor::QmlJSCompletionAssistInterface(
+    return std::make_unique<QmlJSEditor::QmlJSCompletionAssistInterface>(
                 textCursor(), Utils::FilePath(),
                 assistReason, qmljsdocument->semanticInfo());
 }