Implement "holdover events," noting those log events which are not listed in the currently loaded log but which are present because they were playing when this log was loaded.

This fixes problems arising when the duplicate IDs thus introduced cause confusion.

lib/rdlog_line.h

@@ -265,6 +265,8 @@ class RDLogLine
   static QString typeText(RDLogLine::Type type);
   static QString timeTypeText(RDLogLine::TimeType type);
   static QString sourceText(RDLogLine::Source src);
+  bool isHoldover() const;
+  void setHoldover(bool);
 
  private:
   int log_id;
@@ -368,6 +370,7 @@ class RDLogLine
   int log_link_end_slop;
   int log_link_id;
   bool log_link_embedded;
+  bool is_holdover;
 };