split render_pages_nav from render_pager
use pager.items instead of pager._items
add option "instance_index_as_tiles"
Revert "use pager.items instead of pager._items"

This reverts commit 89e6dac6ba3d9006297ee3a118277e07ce7f48d9.

This change implies paging on the page index. We would
rather avoid this for now and eventually do this in a different
pull request.
allow to hide instance index sidebar
Merge pull request #890 from xi/feature-instance-index-as-tiles

instance index as tiles