Replace the draggable.png icon with a modern one, and add up/down-only and left/right-only variants.

This is related to #364 and #2815, as well as a follow-up to #5149.

There's a respective issue in d.org: https://www.drupal.org/project/drupal/issues/216898

This is the current icon:

It is currently being used in the following places in core: - https://github.com/backdrop/backdrop/blob/1.x/core/modules/system/css/sy... - https://github.com/backdrop/backdrop/blob/1.x/core/modules/layout/css/la...

I'd like to propose to replace the current icon with separate black/white variants of https://fontawesome.com/v5.15/icons/arrows-alt?style=solid

I would also like us to add black/white variants of the following icons: - https://fontawesome.com/v5.15/icons/arrows-alt-h?style=solid - https://fontawesome.com/v5.15/icons/arrows-alt-v?style=solid

One use case for the up/down arrow in core is the flexible layouts edit form, where we are currently using the drag cross icon. This doesn't make sense in that page though, since region rows in that form can only be dragged up/down.

Using fontawesome as the source would make it easier to replace these with the respective font icons later (#364), and that's what we have been using for the icons in places like the admin bar, the Dashboard etc.