Support for focal point via Crop API

Problem/Motivation

I would like to be able to use relative crop (for aspect ratio crops) with user-specified focal points.

Steps to reproduce

1. Install image_effects, focal_point
2. Create an image style
3. Try and specify a relative crop using a focal point rather than gravity

Proposed resolution

Create a submodule, or optional dependency, to utilise the crop_api or focal_point modules.
Allow picking a `crop` as well as gravity in the crop effects.

Remaining tasks

Create a submodule.

User interface changes

Provide a focal_point checkbox or crop_api selector for all crop effects.

API changes

Hook into crop_api to retrieve focal points.
Provide a hook for crop effects to retrieve focal points.

Data model changes

None