Issue #3516887 by plopesc, gxleano, m4olivei, pdureau: Allow to customize Navigation icons

...on/components/toolbar-button/toolbar-button.component.yml

.../navigation/components/toolbar-button/toolbar-button.twig

core/modules/navigation/layouts/navigation.html.twig

core/modules/navigation/src/Menu/NavigationMenuLinkTree.php

...dules/navigation/src/Plugin/Block/NavigationLinkBlock.php

.../navigation/src/Plugin/Block/NavigationShortcutsBlock.php

.../modules/navigation/src/Plugin/TopBarItem/PageActions.php

core/modules/navigation/src/ShortcutLazyBuilder.php

core/modules/navigation/templates/navigation-menu.html.twig

...dules/navigation/templates/top-bar-page-actions.html.twig

...gation/tests/navigation_test/assets/icons/radioactive.svg

...es/navigation/tests/navigation_test/assets/icons/star.svg

...avigation/tests/navigation_test/navigation_test.icons.yml

...tion/tests/navigation_test/navigation_test.links.menu.yml

...on/tests/navigation_test/src/Hook/NavigationTestHooks.php

...ation_test_block/src/Plugin/Block/NavigationTestBlock.php

...es/navigation/tests/src/Functional/NavigationIconTest.php