set foucus of button table only if the device has a dpad

so that Kindle Voyage won't show a highlighted button in
confirm dialog.

frontend/device/emulator/device.lua

@@ -9,6 +9,7 @@ local Device = Generic:new{
     isEmulator = yes,
     hasKeyboard = yes,
     hasKeys = yes,
+    hasDPad = yes,
     hasFrontlight = yes,
     isTouchDevice = yes,
 }

frontend/device/generic/device.lua

@@ -16,6 +16,7 @@ local Device = {
     -- hardware feature tests: (these are functions!)
     hasKeyboard = no,
     hasKeys = no,
+    hasDPad = no,
     isTouchDevice = no,
     hasFrontlight = no,
 

frontend/device/kindle/device.lua

@@ -12,23 +12,27 @@ local Kindle2 = Kindle:new{
     model = "Kindle2",
     hasKeyboard = yes,
     hasKeys = yes,
+    hasDPad = yes,
 }
 
 local KindleDXG = Kindle:new{
     model = "KindleDXG",
     hasKeyboard = yes,
     hasKeys = yes,
+    hasDPad = yes,
 }
 
 local Kindle3 = Kindle:new{
     model = "Kindle3",
     hasKeyboard = yes,
     hasKeys = yes,
+    hasDPad = yes,
 }
 
 local Kindle4 = Kindle:new{
     model = "Kindle4",
     hasKeys = yes,
+    hasDPad = yes,
 }
 
 local KindleTouch = Kindle:new{

frontend/ui/widget/buttontable.lua

@@ -68,7 +68,7 @@ function ButtonTable:init()
             self:addHorizontalSep()
         end
     end -- end for each button line
-    if Device:hasKeys() then
+    if Device:hasDPad() then
         self.layout = self.buttons
         self.layout[1][1]:onFocus()
         self.key_events.SelectByKeyPress = { {{"Press", "Enter"}} }