FileManager: don't load already loaded plugins (#6958)

FileManager:init is called by FileManager:reinit, which caused duplicate
loads. Those can't be useful, (for example to re-load a modified plugin
on runtime), as PluginLoader:loadPlugins memoizes loaded plugins.

frontend/apps/filemanager/filemanager.lua

@@ -479,6 +479,7 @@ function FileManager:init()
     table.insert(self, DeviceListener:new{ ui = self })
 
     -- koreader plugins
+    if not self._plugins_loaded then
         for _,plugin_module in ipairs(PluginLoader:loadPlugins()) do
             if not plugin_module.is_doc_only then
                 local ok, plugin_or_err = PluginLoader:createPluginInstance(
@@ -493,6 +494,8 @@ function FileManager:init()
                 end
             end
         end
+        self._plugins_loaded = true
+    end
 
     if Device:hasWifiToggle() then
         local NetworkListener = require("ui/network/networklistener")