[UX] Navigation settings for all devices (#4680)

Before multiswipes and the gesture manager this was impractical on touch-only devices, but no more!

Also includes some minor textual clarifications on some of the settings.

frontend/ui/elements/reader_menu_order.lua

@@ -61,6 +61,13 @@ local order = {
         "battery",
         "mass_storage_settings",
     },
+    navigation = {
+        "back_to_exit",
+        "back_in_filemanager",
+        "enable_back_history",
+        "----------------------------",
+        "invert_page_turn_buttons",
+    },
     network = {
         "network_wifi",
         "network_proxy",