Feature: #108623 - Allow content element restrictions per colPos 

Description 

Backend layouts have been extended with options to allow only configured types of content elements (referencing tt_content.CType with names like "text", "textmedia", "felogin_pi1" and so on) in backend layout columns ( colPos): The two keys allowedContentTypes and disallowedContentTypes add allow and deny lists on column level. These settings can be set with Page TSConfig based backend layouts, Database based backend layouts do not allow configuring this value at the moment, but this will be added soon.

Example for a backend layout with two rows and two columns configured using Page TSConfig:

mod.web_layout.BackendLayouts {
  exampleKey {
    title = Example
    config {
      backend_layout {
        colCount = 1
        rowCount = 2
        rows {
          1 {
            columns {
              1 {
                identifier = main
                name = Main content
                colPos = 0
                allowedContentTypes = header, textmedia
              }
              2 {
                identifier = right
                name = Panel right
                colPos = 1
                allowedContentTypes = my_custom_cta
              }
            }
          }
          2 {
            columns {
              1 {
                identifier footer
                name = Footer
                colpos = 2
                colspan = 2
                disallowedContentTypes = header
            }
        }
      }
    }
  }
}

The implementation adapts the "New content element wizard" to show only allowed (or not disallowed) content elements types when adding a content element to a column. When editing records, the select boxes "Type" and Column position" are reduced to not allow invalid values based on the configuration. Similar logic is applied when moving and copying content elements.

The feature has been created with extension content_defender in mind. This extension by Nicole Hummel has been around for many years and found huge adoption rates within the community. In comparison to content_defender, the core configuration is slightly simplified and the core implementation does not provide the additional content_defender feature to restrict the number of elements per column ( maxitems).

The core implementation supports the content_defender syntax using the arrays allowed.CType and disallowed.CType. With the example below, allowed.CType is internally mapped to allowedContentTypes. When both allowed.CType and allowedContentTypes are given, allowed.CType is ignored.

mod.web_layout.BackendLayouts {
  exampleKey {
    config {
      backend_layout {
        rows {
          1 {
            columns {
              1 {
                allowed {
                  CType = header, textmedia
[...]

Codewise, the PSR-14 event ManipulateBackendLayoutColPosConfigurationForPageEvent has been added. It allows manipulation of the calculated column configuration. It is marked @internal and thus needs to be used with care since it may change in the future without further note: The event is not dispatched as systematically as it should be, but refactoring the surrounding code can probably not be provided with TYPO3 v14 anymore. Extensions like ext:container however must be able to adapt column configuration with TYPO3 v14 already. The decisions was to provide an event, but to mark it internal for the time being, declaring it as "use at your own risk" if you know what you are doing and within extensions that set up proper automatic testing to find issues if the core changes internals.

Impact 

Backend layout columns can now restrict, which content element types are allowed or disallowed inside of it.