[ Wishlist ] Add Wishlist core module

[filrak]

We should move wishlist logic to Core API Wishlist Module (first create it ^^), use it inside core components and unit test core api modules ( read this if you don't know what core api modules are ). Basically this module will be extremely similar to Cart module which can be found in core/api/cart

What should be done

• Logic from Wishlist core components (Wishlist.js, WishlistIcon.js and other where wishlist is used like Header.js) should be moved to core API modules.
• This components should use this module mixins directly instead of duplicating the logic
• Core Wishlist functionalities should be unit tested
• Every exported modules should be properly commented for Intelisense (see Cart module as example)
• Create docs file (see example https://github.com/DivanteLtd/vue-storefront/blob/master/doc/api-modules/cart.md)

Proposed functionalities (ofc I'm open to discussion if you have better ideas)

• loadFromCache - dispatches wishlist/load
• clearWishlist() - dispatches wishlist/clear
• addToWishlist(product) - dispatches wishlist/addItem with produuct as payload
• removeFromWishlist(product) - dispatches wishlist/removeItem with product as payload
• productsInWishlist - computed property returning products in wishlist from vUEX STORE
• ui/openWishlist() - dispatches ui/setWishlist with true payload
• ui/closeWishlist() - dispatches ui/setWishlist with false payload
• ui/isWishlistOpen - computed property returning ui/wishlist state