Fixes anim restarting when returning to screen

This fixes the bug where animations play when you return to the categories screen. This is done by setting timers that are responsible for removing the animations, these can then play out when the product screen is accessed in the background. These timers are cleared and set to new ones whenever either the favourites or add to cart buttons are pressed.

Doing this I also fixed the bug where if you visited a product page, added to cart then pressed back before the animation finished you would get an error screen.

Closes #5 (closed)