Feature: #69602 - Simplify handling of backend layouts in frontend

See forge#69602

Description

To avoid complex TypoScript for integrators, the handling of backend layouts has been simplified for the frontend.

To get the correct backend layout, the following TypoScript code can be used:

page.10 = FLUIDTEMPLATE
page.10 {
  file.stdWrap.cObject = CASE
  file.stdWrap.cObject {
	key.data = pagelayout

	default = TEXT
	default.value = EXT:sitepackage/Resources/Private/Templates/Home.html

	3 = TEXT
	3.value = EXT:sitepackage/Resources/Private/Templates/1-col.html

	4 = TEXT
	4.value = EXT:sitepackage/Resources/Private/Templates/2-col.html
  }
}
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Using data = pagelayout is the same as using as

field = backend_layout
ifEmpty.data = levelfield:-2,backend_layout_next_level,slide
ifEmpty.ifEmpty = default
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