Distinguish editor-originating messages in the editor log

This fades out messages originating from the editor to make messages
printed by the project stand out more.

This also tweaks wording in some editor messages for consistency.

editor/editor_log.h

@@ -74,7 +74,8 @@ public:
 	enum MessageType {
 		MSG_TYPE_STD,
 		MSG_TYPE_ERROR,
-		MSG_TYPE_WARNING
+		MSG_TYPE_WARNING,
+		MSG_TYPE_EDITOR
 	};
 
 	void add_message(const String &p_msg, MessageType p_type = MSG_TYPE_STD);