applets/kicker: hide separators in menus 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 get sorted to the beginning of the list, which looks quite weird. This commit instead hides the separators when sorting alphabetically. Only Kicker is affected by this change; the underlying model providing the items is unchanged.

This is a more focused alternative to plasma-workspace!2708 (closed).

BUG: 465865 FIXED-IN: 5.27.3

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