[Navigation screen] Only show appender for the currently selected block

[adamziel]

Description

This PR updates the navigator to only display a single appender - one related to the currently selected block:

It also proposes the following way of handling the appender for single menu items - this way there is no need for a separate "add nested menu" action:

How has this been tested?

1. Enable the navigation experiment in Gutenberg > Experiments
2. Go to Gutenberg > Navigation (beta)
3. Play with the navigator and confirm the experience feels natural
4. Confirm the navigator in the post editor keeps working the same way as before

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR.

[github-actions]

Size Change: +574 B (0%)

Total Size: 1.13 MB

Filename	Size	Change
build/api-fetch/index.js	3.4 kB	+1 B
build/block-directory/index.js	6.77 kB	+1 B
build/block-editor/index.js	106 kB	+69 B (0%)
build/block-library/index.js	127 kB	-2 B (0%)
build/components/index.js	194 kB	+17 B (0%)
build/compose/index.js	9.31 kB	+2 B (0%)
build/edit-post/index.js	303 kB	+26 B (0%)
build/edit-post/style-rtl.css	5.6 kB	+168 B (3%)
build/edit-post/style.css	5.6 kB	+168 B (3%)
build/edit-site/index.js	15.5 kB	-1 B
build/edit-widgets/index.js	9.34 kB	+1 B
build/editor/index.js	44.8 kB	+120 B (0%)
build/element/index.js	4.65 kB	+1 B
build/format-library/index.js	7.72 kB	-1 B
build/list-reusable-blocks/index.js	3.12 kB	+1 B
build/media-utils/index.js	5.3 kB	-1 B
build/plugins/index.js	2.56 kB	-1 B
build/primitives/index.js	1.5 kB	+2 B (0%)
build/rich-text/index.js	14.8 kB	-1 B
build/server-side-render/index.js	2.68 kB	+4 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
build/a11y/index.js	1.14 kB	0 B
build/annotations/index.js	3.62 kB	0 B
build/autop/index.js	2.83 kB	0 B
build/blob/index.js	620 B	0 B
build/block-directory/style-rtl.css	892 B	0 B
build/block-directory/style.css	892 B	0 B
build/block-editor/style-rtl.css	11.4 kB	0 B
build/block-editor/style.css	11.4 kB	0 B
build/block-library/editor-rtl.css	7.87 kB	0 B
build/block-library/editor.css	7.87 kB	0 B
build/block-library/style-rtl.css	7.72 kB	0 B
build/block-library/style.css	7.72 kB	0 B
build/block-library/theme-rtl.css	684 B	0 B
build/block-library/theme.css	686 B	0 B
build/block-serialization-default-parser/index.js	1.88 kB	0 B
build/block-serialization-spec-parser/index.js	3.1 kB	0 B
build/blocks/index.js	48.1 kB	0 B
build/components/style-rtl.css	19.5 kB	0 B
build/components/style.css	19.5 kB	0 B
build/core-data/index.js	11.4 kB	0 B
build/data-controls/index.js	1.29 kB	0 B
build/data/index.js	8.45 kB	0 B
build/date/index.js	5.47 kB	0 B
build/deprecated/index.js	771 B	0 B
build/dom-ready/index.js	569 B	0 B
build/dom/index.js	3.17 kB	0 B
build/edit-navigation/index.js	8.25 kB	0 B
build/edit-navigation/style-rtl.css	918 B	0 B
build/edit-navigation/style.css	919 B	0 B
build/edit-site/style-rtl.css	2.96 kB	0 B
build/edit-site/style.css	2.96 kB	0 B
build/edit-widgets/style-rtl.css	2.4 kB	0 B
build/edit-widgets/style.css	2.4 kB	0 B
build/editor/editor-styles-rtl.css	425 B	0 B
build/editor/editor-styles.css	428 B	0 B
build/editor/style-rtl.css	4.26 kB	0 B
build/editor/style.css	4.27 kB	0 B
build/escape-html/index.js	733 B	0 B
build/format-library/style-rtl.css	502 B	0 B
build/format-library/style.css	502 B	0 B
build/hooks/index.js	2.13 kB	0 B
build/html-entities/index.js	621 B	0 B
build/i18n/index.js	3.56 kB	0 B
build/is-shallow-equal/index.js	711 B	0 B
build/keyboard-shortcuts/index.js	2.51 kB	0 B
build/keycodes/index.js	1.94 kB	0 B
build/list-reusable-blocks/style-rtl.css	226 B	0 B
build/list-reusable-blocks/style.css	226 B	0 B
build/notices/index.js	1.79 kB	0 B
build/nux/index.js	3.41 kB	0 B
build/nux/style-rtl.css	616 B	0 B
build/nux/style.css	613 B	0 B
build/priority-queue/index.js	789 B	0 B
build/redux-routine/index.js	2.85 kB	0 B
build/shortcode/index.js	1.7 kB	0 B
build/token-list/index.js	1.28 kB	0 B
build/url/index.js	4.06 kB	0 B
build/viewport/index.js	1.85 kB	0 B
build/warning/index.js	1.14 kB	0 B
build/wordcount/index.js	1.17 kB	0 B

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[draganescu]

Tested and LGTM!