efl_ui_spotlight: migrate from active_index to active_element

efl_ui_spotlight: migrate from active_index to active_element

after playing arround with the widget, we found out that it feels quite
weird to have a index, where most of the time you work with widgets.

We might want to add syntax suger in the future to make it easier to
just jump to the next element, or to the previous, but that still is to
be decided.

The event and the communication to the spotlight manager are still left
to be used with the index, reason for this is, that we might need to
fill there an invalid pointer, if a deletion is triggering an animation,
which seems quite weird. That needs further discussing.

Docx have been updated, the sitemarks about the shifting of the
active_index can be removed, as the element is not subject of change
during content adds/deletes.

ref T7991

Reviewed-by: Jaehyun Cho <jae_hyun.cho@samsung.com>
Differential Revision: https://phab.enlightenment.org/D9813