Add more support for debugging plugins.

Added support for plugin debug messages and better error reporting
when something goes wrong during start up of SLPlugin.

Also added more debug output regarding general plugin messages
as well as debug output related to AIFilePicker.

indra/llplugin/llpluginclassmedia.cpp

@@ -674,7 +674,7 @@ void LLPluginClassMedia::receivePluginMessage(const LLPluginMessage &message)
 					mDirtyRect.unionWith(newDirtyRect);
 				}
 
-				LL_DEBUGS("Plugin") << "adjusted incoming rect is: (" 
+				LL_DEBUGS("PluginUpdated") << "adjusted incoming rect is: ("
 					<< newDirtyRect.mLeft << ", "
 					<< newDirtyRect.mTop << ", "
 					<< newDirtyRect.mRight << ", "