elementary: use array instead of list for subchildren

elementary: use array instead of list for subchildren

this safes in the runtime of elm_test round about 9kb (*).
Additionally, using a array here is jumping way fewer times arround in
memory, as we do not need to jump from node to node in a list.

Additionally, this commit replaces a few abort disabler macros with a
error checking macro. (which cleans the log).

*: explanation: we have round about 600 widgets in elm_test, every
widget is normally refered once, every list node has 4 pointer, makes
round about 9600 bytes or rougly 9 KB. So the messured savings are more
or less explaining the reality.

Reviewed-by: Carsten Haitzler (Rasterman) <rasterman.com>
Differential Revision: https://phab.enlightenment.org/D11374

Change-Id: Ib906697a1ba71277e1fbdebedbdc0c05cb44d53a