Plugins: adds PluginMenuInserter helper (#7419)

Allows user-plugins to register themselves
into "More tools" menu.

frontend/ui/plugin/insert_menu.lua

@@ -0,0 +1,21 @@
+local reader_order = require("ui/elements/reader_menu_order")
+local filemanager_order = require("ui/elements/filemanager_menu_order")
+
+-- A "hacky" way to update plugin menu items on-the-fly.
+-- Use: require("plugins/insert_menu").add("my_plugin_menu_name")
+
+-- This piece of logic / table is singleton in the KOReader process.
+-- It provides a way to add a plugin into the "More Plugins" and is useful to
+-- work with contrib/plugins which are not in the core logic of KOReader.
+-- To avoid duplicating the menu item, caller is expected to call the add once
+-- in the KOReader process, usually it's achieveable to rely on the "require"
+-- function in lua.
+
+local PluginMenuInserter = {}
+
+function PluginMenuInserter.add(name)
+    table.insert(reader_order.more_tools, name)
+    table.insert(filemanager_order.more_tools, name)
+end
+
+return PluginMenuInserter