Deeper investigation into scroll and scroll_at

[recursix]

the action scroll_at was recently added since the scroll function seems to only scroll on the page and not on the active object. If the agent need to scroll inside e.g. a list box. Even if the list box was recently selected, scroll only scroll the page, not the list box. At least this is the behavior that was observed in miniwob.click-scroll-list.

What we may want:

• make unit tests for testing scrolling into sub objects
• Verify that the behavior is indeed as described above on other tasks
• scroll should take bid (similar to how scroll_at works with pixel coordinate)
• Fix unit tests with new API.