Prevent tabbing into closed navigation

When the navigation is closed, add "inert" attribute to prevent tabbing
into the sidebar.

Signed-off-by: Vincent Petry <vincent@nextcloud.com>

src/components/NcAppNavigation/NcAppNavigation.vue

@@ -57,6 +57,7 @@ emit('toggle-navigation', {
 	<div id="app-navigation-vue"
 		class="app-navigation"
 		role="navigation"
+		:inert="!open"
 		:class="{'app-navigation--close':!open }">
 		<NcAppNavigationToggle :open="open" @update:open="toggleNavigation" />
 		<slot />