Add zIndex for components (ready)

This fixes the y-axis ticks bug.

Optimizing for enforcing zIndex after all components have rendered will not work because:
1) Calling update on components after changing their z-index will not do anything.
2) Components don't share the same parent container.

Implementation details:
Done using a zIndex mixin that enforces z-Index on update. This means that this logic runs on every component's update call.

Code is replicated across all the components and this is a known issue. We should fix this in the hardening sprint.