Previously the "use gravity" setting lived in the Menu instance rather than MyAvatar.  I'm moving it to MyAvatar so that it can be exposed to JS, but as a bit in a U32 field rather than as its own bool (*).

My plan is to expose a few other avatar motion options to make it easier to control the avatar.  This "obey gravity" feature is the first but there will be more. Soon there will be a "1U << 1" below this line, etc, and I'll move it into one of the libraries so that it is available to ScriptEngine.

(*) I debated whether to add a getFoo()/setFoo() option for every little avatar motion feature but decided to group the ones that make sense together.  Andrzej suggested that the JS API reflect the C++ API, so the JS scripts will also set bits.  The nice thing about this is that JS will be able to set/clear several features in one call.  If we really need the ability to write "MyAvatar.useGravity = true" then we can add that later.