…ontrols.

- checks global styles for uploaded images and applies defaults
- changes the default position from "center" to "50% 50%" so it displays in the controls
- ensures that reset/remove functionality closes the panel and removes image
- do not save already-inherited styles to the block, and don't apply defaults where an inherited style already exists.

…nel.

Add defaults for position '50% 50%' for blocks so it can be displayed in the background panel controls
Pass inherited values to setBackgroundStyleDefaults so as not apply/overwrite inherited styles.
Tighten up logic in setBackgroundStyleDefaults so that background position isn't set when an inherited value is contain and the block style is something else.

… and block supports, always apply the defaults.

…e defaults.