3.0 paginator sort

Following what was discussed on PR #1659 regarding sorting multiple fields.

This PR drops the `direction` parameter and makes `sort` a field-direction array.
However it does not implement sorting multiple fields as of yet. I will send a second PR to cover it as I figured out would be better to discuss changes and check the code changes.

Some things worth mentioning:
- PaginatorComponent
 - Consistent `alias.field` output, for virtual/whitelisted fields it would have only `field`
 - Whitelist support for foreign/associations fields
- PaginatorHelper
 - `sortDir` now has its first argument as `$field`
 - `sortKey` has been removed