[chore] Bring file manager plugin loading in line with reader

This allows for simple testing if self.ui.plugin_name is loaded without complex additional methods in, e.g., PluginLoader.

frontend/apps/filemanager/filemanager.lua

@@ -357,8 +357,10 @@ function FileManager:init()
                 plugin_module, { ui = self, })
             -- Keep references to the modules which do not register into menu.
             if ok then
+                local name = plugin_module.name
+                if name then self[name] = plugin_or_err end
                 table.insert(self, plugin_or_err)
-                logger.info("FM loaded plugin", plugin_module.name,
+                logger.info("FM loaded plugin", name,
                             "at", plugin_module.path)
             end
         end