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/apps/reader/modules/readertoc.lua

@@ -60,10 +60,10 @@ end
 function ReaderToc:onGesture() end
 
 function ReaderToc:registerKeyEvents()
-    if Device:hasKeyboard() then
-        self.key_events.ShowToc = { { "T" } }
-    elseif Device:hasFiveWay() then
+    if Device:hasScreenKB() then
         self.key_events.ShowToc = { { "ScreenKB", "Up" } }
+    elseif Device:hasKeyboard() then
+        self.key_events.ShowToc = { { "T" } }
     end
 end