Feature: #51291 - Synchronized field values in localized records

See Issue #51291

Description

The localized record overlay behaviour has been changed to make localization-rows standalone.

Previously, if fields in TCA columns were set to l10n_mode exclude or mergeIfNotBlank, the localized record overlay did not contain values, and those values were "pulled up" from the underlying default language records.

This has been changed, the DataHandler now copies those values over to the localized record and synchronizes them if the default language record is changed.

As a substitution of the mergeIfNotBlank feature, the new configuration allowLanguageSynchronization has been added. Setting this adds a wizard to single fields and an editor can select if a field of a localized record should be kept in sync with the default language record, or the localized record it was derived from.

A typical configuration looks like that:

'columns' => [
    ...
    'header' => [
        'label' => 'My header',
        'config' => [
            'type' => 'input',
            'behaviour' => [
                'allowLanguageSynchronization' => true,
            ],
        ],
    ],
],

TCA tables that configure the language localization get the field l10n_state added by the schema analyzer which stores a json string with field names and the values custom, parent or source to specify if and from which record a single field gets its value.