Allow configuring filtering attributes on per-block basis

Problem/Motivation

Now we can configure attributes, visible in the Filters block, only globally, which is not very customizable.

Proposed resolution

Instead of the global setting, let's implement a per-block configuration of the visible filters, and the facet count flag.

1. Move the "Customize a list for product product filters" form to the block setting, and represent as a table with columns:
- Product type (the product type name, and "General" for filters like "Availability/In Stock")
- Attribute name
- Show filter
- Show count (enables querying the facet)

2. Integrate into the table the hardcoded filter "In Stock".