project: Factor out a function to check backup project

https://bugzilla.gnome.org/show_bug.cgi?id=739251
mainwindow: Do not use variables that dp not exist

https://bugzilla.gnome.org/show_bug.cgi?id=739251
previewer: Handle the case where sources are added with inpoint != 0

That happens only when clip are splitted right but it was completely
breaking thumbnail display.
pipeline: Do not forget to reset _timeout_async_id when removing the GSource

When the callback of a GSource return False, it means the GSource is
removed. So we need to properly reset the _timeout_async_id in that
case too.
pitivi: Do not serialize the selection group

We do not want it to be serialized, it is really just a temporary
group to help  us manage several elements as one
bin:env: Handle GES Validate test manager
utils:pipeline: Fix seek failure exception message
undo: Wait for elements to be in a track before discovering their children properties

Otherwize if we do it too soon they will not exist yet