A bizarre C++ wart wants us to declare FromStack overloads

before the ThemeMetric templates that use them.  That's broken
and unreasonable, so change this around a bit and make FromStack
(and Push) templates.

Push() takes a bit of a trick.  Some Push overloads push the
actual value: scalars (int, float), RageColor (pushes a table).
Others--most of them--push a reference to a C++ object.  We
want the scalars to have a reference parameter type, so we
don't make extra copies of things like RageColor when we push
them.  We need to pass C++ objects by pointer (we need to
push the actual object's pointer, not a pointer to a copy).
Further, pushing a scalar is a const operation, but pushing
a reference to an object is not.

To do both with the same template, we handle objects with
this slightly odd template:

template<> void LuaHelpers::Push<T*>( lua_State *L, T *const &pObject );

The actual overload (T) is eg. "Actor*"; this fits within the
general prototype, "Push(lua_State *L, const T &object)", giving
us a const reference to a (non-const) pointer to Actor, and we're
conceptually pushing the pointer.

The net effect of this is that 1: what was before compile errors
now becomes link time errors, but 2: these specializations
don't have to be in the headers (except for new ones for
Preference and BroadcastOnChange).

stepmania/src/MessageManager.h

@@ -3,6 +3,7 @@
 #ifndef MessageManager_H
 #define MessageManager_H
 
+#include "LuaManager.h"
 struct lua_State;
 
 enum Message
@@ -201,6 +202,8 @@ public:
 	bool operator != ( const T &other ) const { return val != other; }
 };
 
+namespace LuaHelpers { template<class T> void Push( lua_State *L, const BroadcastOnChange<T> &Object ) { LuaHelpers::Push<T>( L, Object.Get() ); } }
+
 template<class T, int N>
 class BroadcastOnChange1D
 {