Refactoring hardware abstraction

This is a major overhaul of the hardware abstraction layer.
A few notes:

General platform distinction happens in
  frontend/device.lua
which will delegate everything else to
  frontend/device/<platform_name>/device.lua
which should extend
  frontend/device/generic/device.lua

Screen handling is implemented in
  frontend/device/screen.lua
which includes the *functionality* to support device specifics.
Actually setting up the device specific functionality, however,
is done in the device specific setup code in the relevant
device.lua file.

The same goes for input handling.

frontend/apps/reader/modules/readermenu.lua

@@ -6,10 +6,10 @@ local ConfirmBox = require("ui/widget/confirmbox")
 local GestureRange = require("ui/gesturerange")
 local OTAManager = require("ui/otamanager")
 local UIManager = require("ui/uimanager")
-local Device = require("ui/device")
+local Device = require("device")
 local Geom = require("ui/geometry")
 local Event = require("ui/event")
-local Screen = require("ui/screen")
+local Screen = require("device").screen
 local Language = require("ui/language")
 local DEBUG = require("dbg")
 local _ = require("gettext")
@@ -93,7 +93,7 @@ function ReaderMenu:setUpdateItemTable()
     table.insert(self.tab_item_table.setting, {
         text = _("Screen settings"),
         sub_item_table = {
-            Screen:getDPIMenuTable(),
+            require("ui/elements/screen_dpi_menu_table"),
             UIManager:getRefreshMenuTable(),
         },
     })