[Timeline] hidden when start/end option not set

This is due to FEAT #3529:

When the timeline is being initialized, it's visibility is set to hidden, and it's visibility is only set to visible when a rangechange event is fired. But when start/end is not set, rangechange is not fired. Therefore the timeline is still hidden even after it's initialized.

https://jsfiddle.net/zgrvoxb3/