Allow icons for logic properties be defined in the component schema

[amycheng]

As it works now, the icons for property readouts is hard coded in clay-space-edit. The API for this should be more general because we don't want to edit the clay-space-edit every time there's a new kind of logic property.

How it should work:

• there's a property in the logic Clay component that let's the user choose a icon that represents the field (i.e. clock for time, label for tag)
• the user can only use Material Design Icons
• clay-space-edit looks at this property and renders it in the readouts