LSP: add outline support

Fixes: QTCREATORBUG-21573
Change-Id: If579715c84210eb80d258ec944e00f1fac1badbe
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/languageclient/languageclientmanager.h

@@ -75,6 +75,8 @@ public:
 
     static LanguageClientManager *instance();
 
+    static QList<BaseClient *> clientsSupportingDocument(const TextEditor::TextDocument *doc);
+
 signals:
     void shutdownFinished();