Add a back button in BlockVariationPicker if used with BlockPatternSetup

[ntsekouras]

What problem does this address?

If we use in a block the __experimentalBlockPatternSetup and matching patterns are available, the user will be prompted to choose among the patterns. If the specific block has also block variations to be shown and uses __experimentalBlockPatternSetup when the user chooses to start blank, they need a way (a back button) to go to the patterns picker view.

The problem exists for Query block but could apply for other blocks with the same conditions, and was presented by @annezazu with the following video:

query.undo.with.patterns.mov

[annezazu]

@ntsekouras would this by chance apply to the navigation block at all? Asking because I noticed I can't undo under specific circumstances namely if the existing menu I add is empty:

undo.mov

[ntsekouras]

@ntsekouras would this by chance apply to the navigation block at all?

Navigation block is using different components with different implementation, so this is a different issue.

--cc @draganescu