Kicker model: hide separators when sorting alphabetically

Users are able to manually add separator items at custom positions in their menu structures. When they do so, the location of these separators are inherently custom, and only make sense when the menu structure is being displayed in its custom order. When using the option to display everything alphabetically, the separators' custom position no longer exists and any automatic placement becomes nonsensical.

Currently, the separators gets sorted to the beginning of the list, which looks quite weird. This commit fixes that by avoiding even adding separators within category lists into to the Kicker model itself.

BUG: 465865 FIXED-IN: 5.27.3

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