applets/kicker: return more roles explicitly
Reason for the change
Similar to ef0009ad and 510bd959
Kicker's models sometimes return an empty QVariant if the role is irrelevant to the model. This breaks with required bool properties if the ListView is reused with a different model; the empty QVariant does not become false, but maintains the previous value. Main kicker's submenus reuse the listview, so moving e.g. from an app category submenu to Recents or system actions may cause entries there to be inappropriately marked as subcategories, separators, or (in 6.7) as newly installed.
Instead, explicitly return bool false for the roles where this may be an issue.
Test plan
- Place a category with subsubmenus next to Power/Session
- Hover over that subcategory
- Hover over Power/Session, making sure the submenu does not close in between
Screenshots or screen recordings
N/A