Improve product selection widget

[maarcingebala]

Improve the product selection widget that is used across the Dashboard 2.0 e.g.:

• adding products to a draft order
• adding products to collections or categories

Mockup:

[maarcingebala]

To clarify:

• when creating draft orders we add variants
• in case of categories/collections, we add whole products (with all variants included) so the selector might be simpler in that case and include only one-level list items - only the products.