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/readerfont.lua

@@ -142,20 +142,20 @@ function ReaderFont:onGesture() end
 
 function ReaderFont:registerKeyEvents()
     if Device:hasKeyboard() then
-        -- add shortcut for keyboard
-        self.key_events = {
-            ShowFontMenu = { { "F" } },
-            IncreaseSize = {
+        self.key_events.ShowFontMenu = { { "F" } }
+        if not (Device:hasScreenKB() or Device:hasSymKey()) then
+            -- add shortcut for keyboard
+            self.key_events.IncreaseSize = {
                 { "Shift", Input.group.PgFwd },
                 event = "ChangeSize",
                 args = 0.5
-            },
-            DecreaseSize = {
+            }
+            self.key_events.DecreaseSize = {
                 { "Shift", Input.group.PgBack },
                 event = "ChangeSize",
                 args = -0.5
-            },
-        }
+            }
+        end
     end
 end