[orders] "Pre-order" and "In-stock" modes

The whole order management has been improved by clearly distinguishing the two main order modes: pre-ordering and in-stock ordering.

Order taking now starts with defining the order mode (pre-order or in-stock). For showrooms using the "sales catalog" display mode, this is done by selecting the sales catalog first. The order taking steps now become:

For "collections" display mode:
1. Collections administration: defining the collection order mode (pre-order / in-stock)
2. Order create: selecting the order mode (pre-order / in-stock)
3. Add products to order: products are restricted to collections linked to the selected order mode, quantities are checked against stock levels if order is on "in-stock" mode
4. Order administration: order mode is displayed within the order, orders can be searched by order mode 

For "sales catalogs" display mode:
1. Sales catalogs administration: defining the sales catalog order mode (pre-order / in-stock)
2. Order create: selecting the sales catalog
3. Add products to order: products are restricted to the selected sales calalog, quantities are checked against stock levels if sales catalog is on "in-stock" mode
4. Order administration: order mode is displayed within the order, orders can be searched by order mode 

In the web showroom, each collection - or sales catalog as the case may be - clearly displays its order mode (pre-order or in-stock).