Add `scrollable' setting to debiki_dragscrollable.
Add `scrollable' setting to debiki_dragscrollable.

This new setting is a jQuery selector; it decides which elements are
scrollable. This gives more flexibility than the former
`acceptPropagatedEvent' setting, which should no longer be needed --
remove it.

debiki/debiki-server-old,debiki/debiki-server-old,debiki/debiki-server-old,debiki/debiki-server-old