Added IsSongLocked() Lua binding.

To be honest, I'm wondering if this should be in SongManager
instead of or as well as UnlockManager.

Docs/Changelog_sm5.txt

@@ -11,6 +11,7 @@ StepMania 5.0 $next | 2011????
 2011/09/16
 ----------
 * [UnlockManager] Added GetNumLockedSongs() Lua binding. [Wolfman2000]
+* [UnlockManager] Added IsSongLocked() Lua binding. [Wolfman2000]
 
 2011/09/12
 ----------

Docs/Luadoc/Lua.xml

@@ -1573,6 +1573,7 @@
 			<Function name='PreferUnlockEntryID'/>
 			<Function name='UnlockEntryID'/>
 			<Function name='UnlockEntryIndex'/>
+			<Function name='IsSongLocked'/>
 		</Class>
 		<Class base='ActorFrame' name='WheelBase'>
 			<Function name='GetCurrentIndex'/>

Docs/Luadoc/LuaDocumentation.xml

@@ -4163,6 +4163,10 @@
 	<Function name='UnlockEntryIndex' return='void' arguments='int iUnlockEntryID'>
 		Unlocks an entry by index.
 	</Function>
+	<Function name='IsSongLocked' return='int' arguments='Song pSong'>
+		Determines if a song is locked by any means. The number returned determines
+		how the song is locked.
+	</Function>
 </Class>
 <Class name='WheelBase'>
 	<Function name='GetCurrentIndex' return='int' arguments=''>