applets/kickoff: change highlight appropriately when items are clicked

Part of plasma-workspace#97

Otherwise there is no visual feedback when clicking. This commit fixes that so that it has the same visual feedback used in Folder View, which is fully supported by Plasma themes.

This MR is intentionally minimal, only changing as little as needed to improve the UX here. Ultimately it would be best to use the standard Highlight{} component, but that can come afterwards. Many other widgets with hand-rolled highlight effects could benefit from the same treatment. But, one thing at a time. :)

Clicked grid item	Clicked list item