qmlls client: opt out semantic highlighting for QtC 14.x, take 2

When we restart language server, everything is reset including
dynamically registered/unregistered methods but clients aren't
destroyed. Therefore,  we need to register dynamically registered
entities in the Client::start() method rather than the constructor.

Customize the start behavior of qmlls client by applying template
pattern.

Amends df2e55d92a

Task-number: QTCREATORBUG-31148
Change-Id: Ib974500beb46cb79d972756efd0f6c7bbd7a87af
Reviewed-by: Sami Shalayel <sami.shalayel@qt.io>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/languageclient/client.cpp

@@ -1683,6 +1683,11 @@ void Client::setLogTarget(LogTarget target)
 }
 
 void Client::start()
+{
+    startImpl();
+}
+
+void Client::startImpl()
 {
     d->m_shutdownTimer.stop();
     LanguageClientManager::addClient(this);

src/plugins/languageclient/client.h

@@ -224,6 +224,7 @@ protected:
     void handleMessage(const LanguageServerProtocol::JsonRpcMessage &message);
     virtual void handleDiagnostics(const LanguageServerProtocol::PublishDiagnosticsParams &params);
     virtual DiagnosticManager *createDiagnosticManager();
+    virtual void startImpl();
 
 private:
     friend class ClientPrivate;

src/plugins/qmljseditor/qmllsclient.cpp

@@ -66,10 +66,6 @@ QmllsClient *QmllsClient::clientForQmlls(const FilePath &qmlls)
 QmllsClient::QmllsClient(StdIOClientInterface *interface)
     : Client(interface)
 {
-    LanguageServerProtocol::Unregistration unregister;
-    unregister.setMethod("textDocument/semanticTokens");
-    unregister.setId({});
-    dynamicCapabilities().unregisterCapability({unregister});
 }
 
 QmllsClient::~QmllsClient()
@@ -77,4 +73,14 @@ QmllsClient::~QmllsClient()
     qmllsClients().remove(qmllsClients().key(this));
 }
 
+void QmllsClient::startImpl()
+{
+    LanguageServerProtocol::Unregistration unregister;
+    unregister.setMethod("textDocument/semanticTokens");
+    unregister.setId({});
+    dynamicCapabilities().unregisterCapability({unregister});
+
+    Client::startImpl();
+}
+
 } // namespace QmlJSEditor

src/plugins/qmljseditor/qmllsclient.h

@@ -19,6 +19,7 @@ public:
     explicit QmllsClient(LanguageClient::StdIOClientInterface *interface);
     ~QmllsClient();
 
+    void startImpl() override;
     static QmllsClient *clientForQmlls(const Utils::FilePath &qmlls);
 };