GESTimeline: Lock object discovery list

GESTimeline: Lock object discovery list

TimelineFileSource objects are asynchronously discovered with discoverer
with such objects being added to a pendingobjects list. If one were to
remove a layer before an object in said layer had been discovered, a
segfault could occur.

As such, management of the list has been made more robust with the
addition of a mutex and removal of the object from the pendingobjects
list upon layer removal.