InnerBlockSlider: Select block when slide changes & change slide when selected block changes

[mattheu]

This covers 2 small enhancements that improve the integration with the block editor and just make this a bit nicer to use.

1. When changing to a new slide, select that slide block in the block editor state, ensuring the correct block is focused.
2. When selecting a slide block externally, e.g. from the Document Overview panel, that slide should be set as current. Updated to handle selecting inner blocks too. Note a limitation is that it will not change if you select an inner block of an inner block, which would be nice to handle in future.