This fixes [#6996] Sequence Nesting and enables sequence nesting for the
first time :)
The old hack done by Ton prevented eternal loops by preventing Sequence
tracks being added to Sequence scenes.
We now disable "Do sequence" temporarily for the _current_ scene, which
has the same effect but leaves the possibility of sequence nesting
untouched.
Also fixes a warning in editseq (uninitialized variable).
cb7e393307