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/readeractivityindicator.lua

@@ -1,12 +1,12 @@
 local EventListener = require("ui/widget/eventlistener")
-local Device = require("ui/device")
+local Device = require("device")
 local util = require("ffi/util")
 -- lipc
 
 local ReaderActivityIndicator = EventListener:new{}
 
 function ReaderActivityIndicator:init()
-    local dev_mod = Device:getModel()
+    local dev_mod = Device.model
     if dev_mod == "KindlePaperWhite" or dev_mod == "KindlePaperWhite2" or dev_mod == "KindleTouch" then
         require "liblipclua"
         self.lipc_handle = lipc.init("com.github.koreader.activityindicator")