Spacer block: height in pixels field label should be properly associated

[afercia]

The new "Spacer" block has a "height in pixels" field in the sidebar. This label should be properly associated with the field.

• labels are not just for visual purposes
• they give the control a name
• when clicked, they focus the associated control
• there's no need for an aria-label attribute if the <label> element is descriptive enough