NcAppSidebarTabs: fix activating the tab being added

[ShGKme]

Fix: nextcloud/spreed#9345

Steps to reproduce

1. Add tabs A, C to NcAppSidebarTabs
2. Set active B add add tab B

Actual behavior

New tab is added, but not active.

Expected behavior

New tab is added and active.

Fix

Local activeTab should be updated on each new tab registration. Previously it was updated only if it wasn't set before (I have no ideas, why).

Manual testing example

It's a bit specific, I decided not to add it to the documentation.

<template>
	<div>
	        <button @click="active = 'settings-tab', showTabs[1] = true">Activate settings and add settings-tab</button>
		<NcCheckboxRadioSwitch :checked.sync="showTabs[0]">Show search tab</NcCheckboxRadioSwitch>
		<NcCheckboxRadioSwitch :checked.sync="showTabs[1]">Show settings tab</NcCheckboxRadioSwitch>
		<NcCheckboxRadioSwitch :checked.sync="showTabs[2]">Show sharing tab</NcCheckboxRadioSwitch>
		<NcAppSidebar
		        :active.sync="active"
			title="cat-picture.jpg"
			subtitle="last edited 3 weeks ago">
			<NcAppSidebarTab v-if="showTabs[0]" name="Search" id="search-tab">
				<template #icon>
					<Magnify :size="20" />
				</template>
				Search tab content
			</NcAppSidebarTab>
			<NcAppSidebarTab v-if="showTabs[1]" name="Settings" id="settings-tab">
				<template #icon>
					<Cog :size="20" />
				</template>
				Settings
			</NcAppSidebarTab>
			<NcAppSidebarTab v-if="showTabs[2]" name="Sharing" id="share-tab">
				<template #icon>
					<ShareVariant :size="20" />
				</template>
				Sharing tab content
			</NcAppSidebarTab>
		</NcAppSidebar>
	</div>
</template>
<script>
import Magnify from 'vue-material-design-icons/Magnify'
import Cog from 'vue-material-design-icons/Cog'
import ShareVariant from 'vue-material-design-icons/ShareVariant'

export default {
	components: {
		Magnify,
		Cog,
		ShareVariant,
	},
	data() {
		return {
		        active: '',
			showTabs: [true, true, false],
		}
	},
}
</script>

[Antreesy]

Tested with Talk, where we mount "chat" and "participants" tabs when join the call / group conversation