Specifying no <group> data will result in normal retrieval of the list of
media items. Specifying <group>none</group> will result in not doing any
grouping, independent of any conflicting GUI settings. Specifying
<group mixed="true">set</group> will result in grouping movies in the result
list into sets where possible which will lead to a list containing mixed
items (movies and sets). <group>set</group> (and other values) is currently
not supported but is already introduced in anticipation of future features.