Reworks plugin loading and structure

Implements #4358

scripts/plugins/examples/example/web_gui/menu.lua

@@ -0,0 +1,28 @@
+--
+-- (C) 2013-20 - ntop.org
+--
+
+-- Adds a custom page to the ntopng system menu
+
+-- Changes to this script must be applied by reloading the plugins from
+-- http://127.0.0.1:3000/lua/plugins_overview.lua
+
+return {
+  -- The menu entry label
+  label = "example.custom_menu_entry",
+
+  -- The custom script to execute, located into this directory.
+  script = "example_page.lua",
+
+  -- The sort order in the menu. Entries with higher sort_order are shown
+  -- before entries with lower sort order.
+  sort_order = -1,
+
+  -- Information about the menu entry, see page_utils.menu_entries
+  menu_entry = {key = "example_plugin", i18n_title = "Example Page", section = "system_stats"},
+
+  -- Conditionally show or hide the menu entry
+  is_shown = function()
+    return(true)
+  end,
+}