timeline: fix segfaults

don't call the timeline update, when the reference is invalid

https://bugzilla.gnome.org/show_bug.cgi?id=702605
ges-launch: Disable update while loading a timeline

Avoid useless gaps creation/destruction during the parsing of the
project
formatter: Do not disable update in the formatter baseclass

We need to make sure the update are disabled until the project is fully
loaded, let the responsability to the subclasses
timeline: fix segfaults

don't call the timeline update, when the reference is invalid
test: fix test_ges_timeline_remove_track

* unref things at end to avoid segfault
* check with fail_unless for existing gobject for propper result