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

manual/plugins/snake.tex

@@ -29,7 +29,7 @@ end when the snake touches either the borders of the screen or itself.
      & Toggle Play/Pause\\
     %
     \opt{SANSA_FUZEPLUS_PAD}{\ButtonPower}
-    \opt{SAMSUNG_YH92X_PAD,SAMSUNG_YH820_PAD}{\ButtonRec}
+    \opt{SAMSUNG_YH92X_PAD,SAMSUNG_YH820_PAD}{\ButtonRew}
        \opt{HAVEREMOTEKEYMAP}{& }
     & Go to the plugin's menu\\
 \end{btnmap}