LanguageClient: Add option to start server when needed

Until now all servers where startet on creator startup. Change this
default to start server when first file that matches the configured
filter is opened. Additionally add an option that restores the old
behavior of always on servers.

Change-Id: I595e0ede1cf136cc803181377155bcd58ca1a83c
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/languageclient/languageclientmanager.cpp

@@ -231,6 +231,10 @@ void LanguageClientManager::editorOpened(Core::IEditor *editor)
 
 void LanguageClientManager::documentOpened(Core::IDocument *document)
 {
+    for (StdIOSettings *setting : LanguageClientSettings::currentSettings()) {
+        if (setting->m_client.isNull() && setting->m_languageFilter.isSupported(document))
+            setting->startClient();
+    }
     for (Client *interface : reachableClients())
         interface->openDocument(document);
 }