applets/kickoff: Bring back Ctrl+Tab shortcut to switch tabs
Test plan:
Focus handling seems to be fine when switching tabs via shortcut, and the shortcut isn't active while on a search page.
other minor refactoring changes here:
applets/kickoff: Fix null property access in keys handling binding
Fixes the following error:
file:///kde/share/plasma/plasmoids/org.kde.plasma.kickoff/contents/ui/Header.qml:193:
TypeError: Cannot read property 'view' of null