Currently, expression handling in XML is spotting and inconsistent.

Each actor type needs to evaluate attributes; only a few do.  Strings
are handled with the "@foo()" notation.

Instead, after loading an XML file for loading as an actor,
precompile all XML attributes as Lua expressions.

The primary difference to code is:
 - previously, Attr="string" would act as a string if it was being
   read as one.  Now, specify Attr='"string"'.  Attr="@string" is
   a shorthand.  (note that @strings will not be parsed as Lua
   strings, so can not use Lua escape characters)
 - previously, Attr="@func()" would evaluate func(), and the value
   of the attribute would be the return value of the function.  This
   was only supported for a few attributes.  Now, say Attr="func()",
   and this will work for all fields.
 - Attributes of other types are unchanged, except that all attributes
   may be Lua expressions, eg. Width="SCREEN_WIDTH".
 - Attributes names ending with "Command" are special, like metrics.
   Prefix "%" to disable this.

This brings XML handling in line with metric handling.  (This is an intermediary
step; I have another idea that follows from this that should eliminate the annoying
"'foo'"/"@foo".)

stepmania/src/XmlFileUtil.h

@@ -17,6 +17,8 @@ namespace XmlFileUtil
 	RString GetXML( const XNode *pNode );
 	bool SaveToFile( const XNode *pNode, const RString &sFile, const RString &sStylesheet = "", bool bWriteTabs = true );
 	bool SaveToFile( const XNode *pNode, RageFileBasic &f, const RString &sStylesheet = "", bool bWriteTabs = true );
+
+	void CompileXNodeTree( XNode *pNode, const RString &sFile );
 }
 
 #endif