Improve activate response (v1.1)

[stephband]

Currently, active slide is updated at scrollend. This makes the pagination highlighting seem a little delayed.

This is because updating active slide involves measuring things in the DOM, and if measurement is done mid-scroll the scroll can become janky. It may be possible to cache some measurements at scrollstart or touchstart or pointerstart or something, and rely on .scrollLeft on the container to infer the 'new' measurements during scroll. Try it for version 1.0.1.

[stephband]

This commit 8dcc092 improves activation response via navigation, pagination and gesturing, by updating the active slide right as soon as the slide-show is told to scroll to it.