Allow --project to be used alongside the --evaluate CLI parameter

It is useful to be able to load a project and its extensions when using
the --evaluate parameter to execute a particular script.

Also fixed a crash in Document constructor when using the --project
parameter, since it would generally cause a project to get loaded without
there being a DocumentManager.

src/tiled/document.cpp

@@ -47,7 +47,8 @@ Document::Document(DocumentType type, const QString &fileName,
     mCanonicalFilePath = fileInfo.canonicalFilePath();
     mReadOnly = fileInfo.exists() && !fileInfo.isWritable();
 
-    DocumentManager::instance()->registerDocument(this);
+    if (auto manager = DocumentManager::maybeInstance())
+        manager->registerDocument(this);
 
     connect(mUndoStack, &QUndoStack::indexChanged, this, &Document::updateIsModified);
     connect(mUndoStack, &QUndoStack::cleanChanged, this, &Document::updateIsModified);

src/tiledapp/main.cpp

@@ -389,23 +389,20 @@ void CommandLineHandler::evaluateScript()
     for (QString argument = nextArgument(); !argument.isNull(); argument = nextArgument())
         arguments.append(argument);
 
-    ScriptManager &scriptManager = ScriptManager::instance();
-
     static bool initialized = false;
     if (!initialized) {
         initialized = true;
 
-        PluginManager::instance()->loadPlugins();
-
         // Output messages to command-line
         auto& logger = LoggingInterface::instance();
         QObject::connect(&logger, &LoggingInterface::info, [] (const QString &message) { stdOut() << message << Qt::endl; });
         QObject::connect(&logger, &LoggingInterface::warning, [] (const QString &message) { qWarning() << message; });
         QObject::connect(&logger, &LoggingInterface::error, [] (const QString &message) { qWarning() << message; });
 
-        scriptManager.ensureInitialized();
+        initializePluginsAndExtensions();
     }
 
+    ScriptManager &scriptManager = ScriptManager::instance();
     scriptManager.setScriptArguments(arguments);
     scriptManager.evaluateFileOrLoadModule(scriptFile);
 }