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.

plugins/evernote.koplugin/main.lua

@@ -4,7 +4,7 @@ local InfoMessage = require("ui/widget/infomessage")
 local DocSettings = require("docsettings")
 local NetworkMgr = require("ui/networkmgr")
 local UIManager = require("ui/uimanager")
-local Screen = require("ui/screen")
+local Screen = require("device").screen
 local Event = require("ui/event")
 local DEBUG = require("dbg")
 local _ = require("gettext")

plugins/zsync.koplugin/main.lua

@@ -10,7 +10,7 @@ local InfoMessage = require("ui/widget/infomessage")
 local TextWidget = require("ui/widget/textwidget")
 local DocSettings = require("docsettings")
 local UIManager = require("ui/uimanager")
-local Screen = require("ui/screen")
+local Screen = require("device").screen
 local Event = require("ui/event")
 local Font = require("ui/font")
 local ltn12 = require("ltn12")