remove absolete layout elements
Handle visibility event to open/close akno

Tracking `visibility` in the open/close animations is more accurate
since `visibility` truly represent the given state.
Refine transitioning properties

- Defined the specific properties we're transitioning on. That may speed
  up animations and makes things cleaner.

- Fixed visibility transition on the overlay element. It did not animate
  when the akno was closed.

- Updated tests.
refactored styles using sass composition
latest build