bugfix: Flattened in-game pak menu hierarchy for compat with API < 11.

The double-nested submenu in the previous commit created problems with
the gingerbread device I tested on.
front: Pak switches are no longer mappable (use in-game menu instead).

In the Mupen64Plus core, button indices 14 and 15 are denoted
"reserved". In the input-sdl plugin, those button indices are used to
switch the controller paks, and the switching logic occurs inside the
native code. I prefer the simplicity of using a menu rather than making
the switches mappable.
front: Simplified some vibration command syntax.