Add nested settings mechanism with a template configuration and allowedBlock blocks logic.

[jorgefilipecosta]

Description

We extracted this PR from #5452, so we can merge the logic changes without changing UI and UX for now. From the the code point of view the original PR with this changes was already approved, and was waiting UX enhancements.

This PR makes easy to set settings that configure how a given nested area behaves. For now, the only settings implemented are allowedBlocks and template.
"allowedBlocks" restricts what blocks we can nest inside a nesting area.
"template" defines a set of blocks that are "prefilled" inside a nesting area, using the same syntax of CPT templates.

How has this been tested?

For testing purposes change the columns block InnerBlocks component to:

					<InnerBlocks
						template={ [
							[ 'core/paragraph', {
								placeholder: 'Enter column 1 content…',
								layout: 'column-1',
							} ],
							[ 'core/paragraph', {
								placeholder: 'Enter column 2 content…',
								layout: 'column-2',
							} ],
						] }
						allowedBlocks={ [ 'core/button', 'core/paragraph' ] }
						layouts={ getColumnLayouts( columns ) }
					/>

Verify that after the change to the block, the initial two columns have placeholders and that using the inserter we can only use buttons and paragraphs.
This PR can also be tested in cover image of PR https://github.com/WordPress/gutenberg/pull/5452/files as it still contains all these changes.

[noisysocks]

This totally works. Thanks @jorgefilipecosta!

[noisysocks]

This CSS change should be a part of #5452 and not this PR.

[jorgefilipecosta]

Nice catch @noisysocks!

[jorgefilipecosta]

Thank you for the review @noisysocks!

[aduth]

Should we have documented the template prop ?

[jorgefilipecosta]

Hi @aduth nice catch, I will create a PR documenting the missing props of InnerBlocks.