Introduce Device:useDPadAsActionKeys() (#11900)

1. Non-Kindle-specific `hasFiveWay` behavior is changed to `hasDPad and useDPadAsActionKeys`. For now they remain Kindle-specific in practice, unless one sets `useDPadAsActionKeys = yes` in a user patch.
2. With that disambiguation out of the way, `hasFiveWay` itself is further disambiguated into `hasScreenKB` and `hasSymKey`, as per the actual property being used, rather than something that tends to correlate with it. (It needn't be Kindle-specific per se, but non-Kindle devices have equivalent shortcuts with for example `Shift`.)
  Running the emulator with `DISABLE_TOUCH=1` will set `hasSymKey = yes`, which can be tested with right shift.

Closes #11887.

frontend/device/generic/device.lua

@@ -41,9 +41,11 @@ local Device = {
     hasAuxBattery = no,
     hasKeyboard = no,
     hasKeys = no,
-    hasFiveWay = no,
+    hasScreenKB = no, -- in practice only some Kindles
+    hasSymKey = no, -- in practice only some Kindles
     canKeyRepeat = no,
     hasDPad = no,
+    useDPadAsActionKeys = no,
     hasExitOptions = yes,
     hasFewKeys = no,
     hasWifiToggle = yes,