kcms/lnf: Simplify theme contents selection pages

Some small refactoring to reduce a bit of code duplication.

The checkbox for the "Include Desktop Layout" can be included in the model of the Repeater below, without need for a special case

Specially do not make items visibility in SimplePage depend on one another, since that's usually more confusing and error prone.

On top of !2482 (merged), purposefully detached from that to maybe honor the soft freeze 5.27