Menu: Actually hide PlatformNav if !hasKeys

Returning {} just leads to a present but blank entry

(Also, nobody notice that the capcheck was wrong inside platnav anyway ;p).

frontend/apps/filemanager/filemanagermenu.lua

@@ -465,7 +465,9 @@ To:
     end
 
     -- settings > Navigation; this mostly concerns platform-specific features or physical keys, and applies *everywhere*
-    self.menu_items.platform_navigation_features = require("ui/elements/platform_navigation")
+    if Device:hasKeys() then
+        self.menu_items.platform_navigation_features = require("ui/elements/platform_navigation")
+    end
 
     -- settings tab - Document submenu
     self.menu_items.document_metadata_location_move = {

frontend/apps/reader/modules/readermenu.lua

@@ -241,7 +241,9 @@ function ReaderMenu:setUpdateItemTable()
         self.menu_items.page_turns = require("ui/elements/page_turns")
     end
     -- While also related to page turns, this mostly concerns platform-specific features or physical keys, and applies *everywhere*
-    self.menu_items.platform_navigation_features = require("ui/elements/platform_navigation")
+    if Device:hasKeys() then
+        self.menu_items.platform_navigation_features = require("ui/elements/platform_navigation")
+    end
     -- insert DjVu render mode submenu just before the last entry (show advanced)
     -- this is a bit of a hack
     if self.ui.document.is_djvu then

frontend/ui/elements/platform_navigation.lua

@@ -1,15 +1,9 @@
 local Device = require("device")
-
--- Everything in there requires physicla page turn keys ;).
-if Device:hasKeys() then
-    -- No menu entry at all if we don't have any
-    return {}
-end
-
 local Event = require("ui/event")
 local UIManager = require("ui/uimanager")
 local _ = require("gettext")
 
+-- This whole menu is hidden behind a hasKeys device cap.
 local PlatformNav = {
     text = _("Page turn behavior"), -- Mainly so as to differentiate w/ "Page Turns" when in readermenu...
     sub_item_table = {