[KItemListView] Check if KItemListStyleOption actually changed before emitting a change

This avoids work being done when it doesn't need to be.
For instance, the preview generator waits for everything to have settled using a 200ms timer before generating a preview.
This timer fired also in response to onStyleOptionChanged and needlessly delayed preview generation when navigating between folders
despite the style option (e.g. icon size, view mode) not having changed.

Differential Revision: https://phabricator.kde.org/D11481