applets/kicker: make runner columns horizontally scrollable
When additional runners are enabled and there are hits from many of them, kicker's runner columns can easily overflow. This makes some of them inaccessible and looks as broken as it is (including potentially escaping the representation width.
Instead we limit the maximum width by screen width, and make the whole thing horizontally scrollable if the content is larger than that. Keyboard navigation is adjusted to make the focused element visible, mouse use does not scroll by itself (as that is very disorienting).
Screenshots are with runner minimum sizes doubled for testing (ItemListView.qml, L18) - I somehow don't get bookmarks synced here, and without those it rarely overflows for me.
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