YH8xx,YH9xx: Keymap improvement

The main "innovation" in this patch are two "virtual buttons"
for the record switch on YH92x targets. When the switch state
changes, a single BUTTON_REC_SW_ON or .._OFF button event will
be generated. Thus keymap code can react on switching, but
not on the actual state of the switch.

Wherever sensible, the following user scheme is applied:
- use PLAY as confirm button
- use REW button or Long REW to exit
- use REC (YH820) or FFWD (YH92X) as modifier key for button combos

Change-Id: Ic8d1db9cc6869daed8dda98990dfdf7f6fd5d5a1

apps/plugins/chessbox/chessbox.c

@@ -680,6 +680,10 @@ static struct cb_command cb_getcommand (void) {
 #ifdef CB_PLAY_PRE
                 if (lastbutton != CB_PLAY_PRE)
                     break;
+                /* fallthrough */
+#endif
+#ifdef CB_PLAY_ALT
+            case CB_PLAY_ALT:
 #endif
                 result.type = COMMAND_PLAY;
                 return result;