[Feature] Additional pool element offset methods

[acki02]

This is a suggestion to add new pool elements with adjustable offset methods to the origin of sub-pieces in jigsaw structures.

The theoretical implementation could look as follows:

• Additional pool elements formations:simple_pool_element and formations:stacked_pool_element, with element type field, offset tag field and additional fields depending on type.
• offset tag field has an offset_type, and additional fields depending on type.
  • offset_type could be set to none, static or projection
  • offset_type:none has no additional fields.
  • offset_type:static has offsetFront, offsetUp and offsetSide tag fields with max and min fields within each.
  • offset_type:projection has surface, shift, limit_celling and limit_floor fields, and optional offsetFront, offsetUp and offsetSide tag fields with max and min fields within each.
• formations:stacked_pool_element has additional elements tag list and stack_method field.
  • this element type creates a continuus line of pool elements from origin point to offest point.
  • stack_method could be set to grow, connect, grow_best and connect_best, controlling stack origin point, and element selection.

Please ask questions if additional clarifications as to the intent of any parts of this suggestions are needed.