Use a local array of characters to handle control focus changes
Draw Characters by a manually defined order.

It kind of beats the purpose of the Level inheriting Group,
but the main reason behind this Actor#setZIndex doesn't
work as (I) expected.

Another caveat may be when actors other than Characters
are added to the Level.
Merge pull request #52 from rafaelgonzalez/characters-draw-order

Characters are drawn in the proper order