core: Implement "center of gravity" option for crops

No frontend yet, just the logic for specifying the cropping coordinate.
(cherry picked from commit e470c6d5d9a98d1c9cce7263241415e2a6d0d2f2)
mod_admin: Create interface to pick the cropping center on images.
(cherry picked from commit fe0b04a0e4dee3c832b110fc5adab38239a7980a)

Conflicts:
	modules/mod_admin/translations/nl.po
core: Fixes to image cropping with a center coordinate

The cropping point is now actually the center point around the crop
coordinate, not the top-left.

When 'crop' does not have any arguments and a resource id is given in
the image tag, it will load the `crop_center` property from the
resource and use this as the cropping center.

Also updated the documentation
(cherry picked from commit 798e05684657a9c6defcddb32abd1129fb14af59)