examples: Fix applets

most interesting changes:

Fix some component ids

On Shadows page I removed a grammatically invalid quoted id, so there is no way this component ever worked since it was committed to git.

Port some examples to Plasma 6, improve their UI

This porting is not complete: there are bigger applets at the end of their alphabetic list. But before completing the porting, we should decide whether they are still needed at all. They've been broken for ages without anyone being vocal about it.