Extract strings to constants

[franmomu]

Subject

Some polishing, there are multiples places where these strings are used, this replaces the scalar strings by constants.

Since they seem related to Datagrid, I've created them in DatagridInterface. I've chosen KEY_ prefix, but any other name is welcome.

I am targeting this branch, because these changes are BC.

Changelog

### Added
- Added `DatagridInterface::SORT_ORDER` constant to use it instead of `_sort_order` string.
- Added `DatagridInterface::SORT_BY` constant to use it instead of `_sort_by` string.
- Added `DatagridInterface::PAGE` constant to use it instead of `_page` string.
- Added `DatagridInterface::PER_PAGE` constant to use it instead of `_per_page` string.

[VincentLanglet]

Seems nice

[jordisala1991]

I think we are using as a key of an array is an implementation detail, what about just removing the prefix?

[core23]

You can use https://github.com/doctrine/collections/blob/1.6.x/lib/Doctrine/Common/Collections/Criteria.php#L18 for ASC and DESC

[franmomu]

Not sure about this, the phpdoc of the class says:

/**
 * Criteria for filtering Selectable collections.
 */

and the usage inside the doctrine organization: https://github.com/search?q=org%3Adoctrine+Criteria%3A%3ADESC&type=Code looks like it's only around the Criteria class.

[jordisala1991]

Thnk you @franmomu