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/llpluginmessage.h

@@ -100,6 +100,13 @@ public:
 	// Returns -1 on failure, otherwise returns the number of key/value pairs in the message.
 	int parse(const std::string &message);
 
+	enum LLPLUGIN_LOG_LEVEL {
+		LOG_LEVEL_DEBUG,
+		LOG_LEVEL_INFO,
+		LOG_LEVEL_WARN,
+		LOG_LEVEL_ERR,
+	};
+
 	// For debugging purposes.
 	friend std::ostream& operator<<(std::ostream& os, LLPluginMessage const& message) { return os << message.mMessage; }