CrmUI - Improve crmUiEditable to support placeholders, and use its own css

CrmUI - Improve crmUiEditable to support placeholders, and use its own css

Now this widget supports having a `placeholder`.
Also gave it its own css instead of piggybacking off crm-editable classes,
as that was a bit messy and Shoreditch was applying some extra stuff which
then had to be undone.