Generalize KML importer to create any asset type

Problem/Motivation

The KML Importer (provided by the farm_import_kml module) only supports creating Land assets.

This is a bit of a legacy rollover from its origin in the farm_area_import module in farmOS 1.x, which was used to create Areas. Areas are Assets now in 2.x, and most area types are migrated to Land assets with a new "Land type" field option. So the new KML Importer provides options for selecting the "Land type" of new assets, in the same way that the Area Importer provided an option for selecting the "Area type".

The only difference now is that we also have Structure (building/greenhouse) and Water assets. It is not possible to create these via the KML importer, nor is it possible to create other asset types (eg: Forest assets provided by the Forest module → ).

@paul121 added an @todo that describes the challenge:

 * @todo Allow placemarks to be imported as any asset type.
 * This is challenging because some asset type bundles have required fields
 * like the "plant type" and "animal type".

Proposed resolution

TBD

Remaining tasks

TBD

User interface changes

TBD

API changes

None.

Data model changes

None.