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.
Merge branch 'holdover-fix' of https://github.com/smowton/rivendell into smowton-holdover-fix
2014-09-24 Fred Gleason <fredg@paravelsystems.com>
	* Applied a patch from smowton that fixed a clashing id problem
	when auto-refreshing logs in RDAirPlay [GitHub pull request #000028].
Merge branch 'smowton-holdover-fix'