Remove the hideIntro override and default preview modal to false

The use of an "override" prop that allowed the preview modal to be hidden without actually changing the redux state was causing a problem where the block editors where not closing when modals were opening. That is because it relied on the modal reducer being accurate.

I defaulted the preview state to false to make it simpler to consume the GUI, as www will not want the preview info to be true. I made it true in the playground instead.

This fixes https://github.com/LLK/scratch-gui/issues/3710