FieldLayout, styling improvements and standardization

FieldLayout
- Added FieldLayout which provides label and layout features to a widget
- Made Fieldset support adding FieldLayout items
- Removed CheckboxWidget (not needed now, just place a checkbox in a
  FieldLayout and set 'align' to 'inline'
- Updated demo to use FieldLayout
- Moved input cleverness of InputLabelWidget to FieldLayout, and renamed
  it to LabelWidget

Styling improvements
- Made ButtonGroup and ButtonSelect widgets never wrap
- Made Fieldsets use IconedElement mixin instead of rolling its own icon
  support

Standardization
- Added support for passing 'items' config option to most widgets that
  mixin GroupElement (not all should have this, so it's not done at the
  top level)
- Updated all uses of mixins to include final config argument allowing
  config options to be added to mixin classes without updating all callers
- Added config option to clippable element for future use
- Renamed 'options' to 'config' in Toolbar

Change-Id: I87e1bdf5ccb67ce58022926c4faede0c70a1f4a4