LanguageClient: move document symbol support check to client

Used there to prevent requesting document symbols from server that do
not support these requests.

Change-Id: Icca64c41415ba7098c769854823431fb05e5ce1d
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/languageclient/client.h

@@ -193,6 +193,7 @@ public:
     void setSnippetsGroup(const QString &group);
     void setCompletionAssistProvider(LanguageClientCompletionAssistProvider *provider);
     void setQuickFixAssistProvider(LanguageClientQuickFixProvider *provider);
+    virtual bool supportsDocumentSymbols(const TextEditor::TextDocument *doc) const;
 
     // logging
     enum class LogTarget { Console, Ui };