small changes to menu and addition of a reader menu

the reader menu is still very bare-bone, I plan to enhance it now.

frontend/ui/reader/readermenu.lua

@@ -0,0 +1,24 @@
+ReaderMenu = InputContainer:new{
+	key_events = {
+		ShowMenu = { { "Menu" }, doc = "show menu" },
+	},
+}
+
+function ReaderMenu:onShowMenu()
+	local item_table = {}
+
+	table.insert(item_table, {
+		text = "Return to file browser"
+	})
+
+	local main_menu = Menu:new{
+		title = "Document menu",
+		item_table = item_table,
+		width = 300,
+		height = #item_table + 3 * 28
+	}
+
+	UIManager:show(main_menu)
+
+	return true
+end