Clean up Edit: rename EditorDecorationView to FieldDecorationView

After manual testing, everything continues to work fine.

Blocked on #2133869: Clean up Edit's data- attributes.

#5: IIRC, the reasoning behind "EditedElement" rather than "EditedField" is because in the case of a plain text field, the decorated element is not the field, it's a specific item of the field.

A field is always rendered like this:

<div class="field-wrapper">
  <div class="field-item">
    VALUE
  </div>
</div>

There may be many .field-items. Usually, the edited element is the .field-wrapper (i.e. "the field"). But in the case of in-place editing a single-valued textual field (whether plain or rich text), the edited element is .field-item (i.e. "a field item").

However, I see your point. Maybe it's better to persist the frequently occurring analogy of "if the logic exists for the Entity, it also exists for the Field" in Edit, in which case FieldDecorationView would actually be the most fitting name. Especially because it would be the perfect analogy for #2134929: Clean up Edit: rename EntityView to EntityDecorationView.

And this should go in after #2135689: Clean up edit.module to match coding standards.

Okay, done :)

Now also backported to the Drupal 7 contrib module: http://drupalcode.org/project/edit.git/commit/8e34208.