Update shopping cart example

[hootlex]

Checkout related mutations

• The mutations that update the checkoutStatus can be independent of the ones that alter the cart items.
• Added the SET_CART_ITEMS and SET_CHECKOUT_STATUS mutations and removed the CHECKOUT_* ones to make debugging easier.

Cart module getters

• The cartProducts could live within cart module’s getters instead of the root getters.
• The total price of the cart should be a getter instead of a computed property.

Abbreviations

Replace the p variable name abbreviation with product to make the example easier to read.

[chrisvfritz]

@hootlex Very well-described changes and nice, incremental changes! I don't see anything that isn't an improvement. @ktsn What do you think?

[ktsn]

LGTM. Great job!