Currently all metrics except for strings and commands are valid

Lua expressions, and commands simply allow a shorthand that generates
valid Lua expressions.  Make strings Lua expressions.

This essentially reverses strings; rather than:
  Metric1=Text
  Metric2=@Text()

specify:

  Metric1="Text"
  Metric2=Text()

This means that every metric (except Fallback, currently) can be
treated as a Lua expression, except for Command metrics that just
need a bit of preprocessing.

stepmania/src/ThemeManager.cpp

@@ -888,9 +888,11 @@ RString ThemeManager::GetMetric( const RString &sClassName, const RString &sValu
 {
 	RString sValue = GetMetricRaw( g_pLoadedThemeData->iniMetrics, sClassName, sValueName);
 
-	EvaluateString( sValue );
+	RString sRet;
+	LuaHelpers::RunExpressionS( sValue, sRet );
 
-	return sValue;
+	FontCharAliases::ReplaceMarkers( sRet );
+	return sRet;
 }
 
 void ThemeManager::EvaluateString( RString &sText )