Convert form static to dynamic product fields

[Zen4All]

Currently all product fields are hard-coded in the product collect_info page, and there is a lot of code duplication for each product type. This is a pain for those people writing mods and need to add extra fields to the page.

Time for a change, and try to make this easier.

After much trial and error, I came to this solution:

• Place all the fields in separate files
• Use a database table to save which fields are to be shown in a product type
• Put new fields in new tables PRODUCTS_EXTRA, and PRODUCTS_EXTRA_DESCRIPTION (multi language fields) Decided not to do this for better compatibility with existing extra fields
• create new fields
• delete fields
• Add product types
• Edit product types
• Remove product types
• Add tabs
• Edit tabs
• Remove tabs
• Load fields in collect info
• Load/build database query from fields
• Load language defines
  Parts of this are based/inspired on the Numinix product fields module.