pat-contentbrowser: unused and missing pattern arguments

While trying to customize the SelectedItem component of the contentbrowser app, I recognized the following:

• The selection-template argument is nowhere used. It can probably removed.
• The layout Svelte property is not defined in the parser, but documented and used in the code. It cannot be configured via pattern options and need to be added to the arguments.
• The custom-component-keys argument is documented, but in code it is used as componentRegistryKeys. This maybe should be aligned to the documentation, as I think the naming of custom-component-keys is more clear than component-registry-keys. Is is it already used in production and therefore the documentation should be adapted? We can also add an alias, so there is a BBB compatibility with old code.