add display_prices in variation data

The commit introduces 3 changes:
1. It adds 2 extra fields to the available_variations array,
display_price and display_regular_price, to be used by extensions in
cases like
https://github.com/woothemes/woocommerce-product-addons/issues/19
2. Introduces a get_display_price() function in WC_Product to retrieve
product prices incl or excl tax relative to the
woocommerce_tax_display_shop setting.
3. Fixes a minor issue where passing a 0 price to
get_price_including_tax() or get_price_excluding_tax() results in a
wrong output.
Added the possibility to display notices and/or force reload on order review.
set timezone_name_from_abbr 3rd arg to 0

Closes #6897
Allow a cancelled order to be marked paid, if there is a payment delay

Closes #6810
Fixed API products when save multiple images from the media library, closes #6792
Show the shipping calculator only in cart page
[Tests] removed deprecated tax method
Merge pull request #6894 from franticpsyx/add_shop_prices_in_variation_data_array

Add display_prices in available_variations array
[Tests] Fixed the tests for wc_timezone_string()
[API] Added parent_id in products endpoint, closes #6786
Make custom event 'woocommerce_variation_select_change' for theme developers as last thing we trigger. #6833
Use post type label for "Save Order" button

To improve compatibility with custom order types

Conflicts:
	includes/admin/meta-boxes/class-wc-meta-box-order-actions.php
Display order filters for custom order types

That declare they are using order-meta-boxes (and therefore, are assumed
to be exposing the Edit Order interface and a list table)
Change insert button label for custom order types
Apply request filters for custom order types
Delete order items for custom order types
Make no order notes label more generic

To account for it being displayed on custom order type Add/Edit screens
where the order type is not referred to as an 'Order' (e.g. a 'Subscription',
'Refund' or 'Pre-order').
Use order type's singular label for meta box names
Give child class access to billing/shipping fields

So that custom order types can extend `WC_Meta_Box_Order_Data` for their
own purposes without having to redefine all the billing/shipping fields
Use wc_get_order() not new WC_Order()

Because an order might be a custom order type