Feature: #99212 - Group select item in FormEngine via TSconfig

See forge#99212


The existing TSconfig feature TCEFORM.{tablename}.{fieldname}.addItems can now be used to add new items into existing select item groups by using the .group sub-property set to the group identifier. This grouping is usually shown in select fields with groups available.


When using the TSconfig addItems feature, the group property can now be used:


TCEFORM.tt_content.layout.addItems {
    new-layout = My new layout
    new-layout.icon = icon-identifier
    new-layout.group = special