Abstract relational fields view modes

[ryansupercool]

We use Cp::elementSelectFieldHtml within a custom FieldLayoutElement. We'd like to add support for the new view modes coming in 5.9 so developers can configure how these elements display. To make this easier, it would be great if:

1. The view mode constants e.g. BaseRelationField::VIEW_MODE_LIST were moved outside BaseRelationField, perhaps to an Enum.
2. The settings HTML currently within BaseRelationField::getViewModeFieldHtml() was moved to a Twig template/made available under the Cp helper e.g. Cp::viewModeSelectHtml().

The latter isn't strictly necessary, and we could get away with a dropdown, but it would be nice to have the icon based UI used elsewhere without having to duplicate it.