[!!!][FEATURE] Parent/child relationships with data inheritance by product type

[mabolek]

Fixes #224.

• Products use a parent/child relationship built around a Product's parent property.
• The Product sub_product property is replaced by a virtual (non-persisted) children property, which relies on the parent.
• The above changes removes the reliance on an MM table for product variants/sub product, reducing the number of table joins and improving performance.
• When saving a Product record in the Backend, all child products will automatically be updated with data from inherited properties. This is similar to TYPO3's inherited non-localized properties. Although it slows down save operations in the backend, this is a performance boost for read operations in the frontend, as no overlays need to be processed during rendering.
• When editing a child product, inherited properties are not editable and include an information text ("Inherited from parent product.")
• When editing a child product, editable fields display the parent product's value below the input field.
• In the Product edit form, fields in the Relations tab have been migrated from type select to group. This is a performance boost in large installations because a list of all available products is no longer loaded with the field.
• Removed showRecordFieldList in TCA because it is no longer used by TYPO3 (deprecated and ignored).
• Attributes are now using a real TCA-defined inline relation with full support for inheritance (the inline styling is modified using CSS to make them look like normal fields). This removes support for attribute sets in different tabs (at least temporarily), but it resolves a number of issues relating to custom data persistence functions. Everything is now using DataHandler, datamap, and cmdmap for full TYPO3 core support (including correct relation management and reference table indexing).

Not implemented, but a good idea for later:

• Button in the doc header for "Create new child product", which automatically creates a product with parent set (and inherited values updated).

Edit product type: Select fields to be inherited by child products.

Inherited properties are not editable and non-inherited properties display the value for the parent product below the input field.

Fields in the Relations tab have been migrated from type select to group.

Attributes are now using a real TCA-defined inline relation with full support for inheritance.