Updated the update loop to dissociate renderable objects and non renderable ones

Any objects that now defines a draw method MUST inherit at least from
`me.Renderable`
Merge branch 'master' into 1.0.0-dev
renamed `getEntityByProp` to `getChildByProp`

- `getChildByProp` now also return any type of matching object, and not
anymore only `entities` type
` `isEntity` has been removed as well