Fix: drawer not appearing on configured side

[jramrath]

See #2875

[jramrath]

Turns out we can't remove the transition_type handling. I've added this comment:

/* The transition direction of a drawer is not actually determined by the transition type,
 * but rather by the order of 'box' and 'revealer_box':
 *   'REVEALER_TRANSITION_TYPE_SLIDE_LEFT' and 'REVEALER_TRANSITION_TYPE_SLIDE_RIGHT'
 *   will result in the same thing.
 * However: we still need to differentiate between vertical and horizontal transition types.
 */

[Alexays]

LGTM thx.