Field show/hide options for stock fields and variations
Product admin/saving
Simplify get_children and use stock status rather than stock level
Conditional is no longer required
Add sanity check to stock status change
Only call the method to update stock when it changes
Hide status box for variables and handle save event
Only use a value for changes - allows automatic changes to stock status to take over
Static is required
Handle stock for self and children, as well as syncing status with the children
Variation class modifications (see details)

- Made it load data on the fly, rather than loading it all at once
- Pull some data from variation, some from parent
- Updated stock methods to use variation or parent depending on
manage_stock
- Ensure parent stays in sync on stock status change event
Data update
Added wc_stock_amount() core function and replaced intval

#4612
Merge branch 'variation-stock-management'

Conflicts:
	includes/class-wc-product-variation.php