Background Media Plugin: Type Image: Add Background Position feature

Problem/Motivation

We would like the ability to set the position of our background media within the section. Currently it is being anchored to the top left.

Steps to reproduce

Add a background media element and inspect the position.

Proposed resolution

Add a new feature to allow for controlling the position of the background.

Remaining tasks

n/a

User interface changes

A new selector for choosing the anchor point for the background image.

API changes

n/a

Data model changes

n/a