feat(simpleplugin) Switch DateAttribute to html5 input type=date

This replaces our custom datepicker with the standard html5 one for date
 fields in plugins.
This deprecates the defaultDate attribute which was used to set the
 starting point of the datepicker without setting a default value,
 because this is not possible in HTML5.
This does not add support for min and max dates which are supported by
 the HTML5 input type and may be added later.
This also changes the internal date format from DD.MM.YYYY to YYYY-MM-DD
 to match the HTML5 POST format, this should not have serious
 consequences.

issue #5931