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

Description

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.

Impact

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

Example:

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