Allow WC_Product_Variation children to extend meta

When SHA: 50a0577b81 introduced `$variation_level_meta_data` and
`$variation_inherited_meta_data`, it set their access to `private`.
Child classes of `WC_Product_Variation` should be able to extend these
properties to allow them to take advantage of the parent's `__isset()`
and `__get()` methods.

Related to #6065
If other notices are closed this translation won't have stylesheet so
Fix text-decoration issue with notice in themes.php
Updated with text-decoration
Updated with grunt dev
Order totals: 4 new action hooks
Only display "Refund" when unrefunded amount > 0

If an order has been fully refunded, the "Refund" button was still
displayed despite there being no amount to refund.
Merge pull request #6128 from thenbrent/fix_refund_button

Only display "Refund" when unrefunded order amount > 0
Merge pull request #6127 from tamarazuk/master

Order totals: 4 new action hooks
Merge pull request #6125 from axisthemes/notices

Better Notices in themes.php
Merge pull request #6120 from thenbrent/protected_variation_meta_data

Allow WC_Product_Variation children to extend meta
Sanitize taxonomy names Closes #6124
Add second orderby parameter when sorting by price (ID)

Fixes #6121
Hide export csv button when browser lacks support

Closes #6122