Feature: #66370 - Add flexible Preview URL configuration

Description

It is now possible to configure the preview link generated for the save+view button in Backend.

This allows to have different preview URLs depending on the record type.

Common usecase is to have previews for blog or news records, but this feature now allows you to define a different preview page for content elements as well, which might be handy if those are stored in a sysfolder.

Impact

New page TSconfig is introduced. The options are:

TCEMAIN.preview {
	<table name> {
		previewPageId = 123
		useDefaultLanguageRecord = 0
		fieldToParameterMap {
			uid = tx_myext_pi1[showUid]
		}
		additionalGetParameters {
			tx_myext_pi1.special = HELLO # results in tx_myext_pi1[special]
		}
	}
}

The previewPageId is the uid of the page to use for preview. If this setting is omitted the current page will be used. If the current page is not a normal page, the root page will be chosen.

The useDefaultLanguageRecord defaults to 1 and ensures that translated records will use the uid of the default record for the preview link. You may disable this, if your extension can deal with the uid of translated records.

The fieldToParameterMap is a mapping which allows you to select fields of the record to be included as GET-parameters in the preview link. The key specifies the field name and the value specifies the GET-parameter name.

Finally additionalGetParameters allow you to add arbitrary GET-parameters and even override others.