Based on patch from Alexander Kuznetsov. Own changes:
- Keyframes in timelime depends on active object, so timelime better be
listeing to ND_OB_ACTIVE notifier rather than ND_OB_SELECT
- When scene is changing in this operator NC_WINDOW notifier would be send
and the whole interface would be redrawed, so no need in ND_OB_ACTIVE in
this case