Update styles and keyboard accessibility

Keyboard focus on a range input now causes the value bubble to appear.

Added a .disabled class for labels which fades the text color.

Removed the left margin from checkboxes and radio buttons at the
beginning of a label.
Rename setting updated event

Changed the name of the CustomEvent fired when a setting changes from
'settings' to 'setting' to reflect that only one setting is being
changed.

If you use this event, you will need to change the name in your code to
properly bind to the event again.