Change inserter inside InnerBlocks.

[herregroen]

Currently the inserter used inside InnerBlocks is the same as the general one used to edit posts.

When core/paragraph is allowed inside InnerBlocks this works for adding paragraphs but adding other types of blocks can be confusing as this depends on which block is currently active.

When core/paragraph is not allowed inside InnerBlocks adding blocks doesn't work at all as there's no way to make the InnerBlocks active.

To solve these I would suggest the default inserter for InnerBlocks is a simple plus that's always visible, clicking on it opens the popover which displays only a list of allowedBlocks. This is, by default, the only way to add blocks to InnerBlocks in order to remove all confusion.

Optionally a prop may be passed to revert back to the previous inserter if the user of InnerBlocks feels that makes sense.

[ZebulanStanphill]

Dupicate of #6834. See also: #6569, #7006, and #7763.

[karmatosed]

Closing as we are collating into one issue.