Update GUI verifier description with new buttons

The screenshot on [Verifier](https://docs.uppaal.org/gui-reference/verifier/) and the description on [Specifying requirements](https://docs.uppaal.org/gui-reference/verifier/specifying/) still mention the Uppaal v4 buttons. In Uppaal v5, the button `insert` is replaced by `Insert above` and `Insert below`, and a new button `Clear results` is added.