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

See Issue #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
  }
}

Using data = pagelayout is the same as using as

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