applets/kicker: Hide separators when sorted 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.

BUG: 465865
FIXED-IN: 5.27.4
(cherry picked from commit 90ad64ba)