Forward ref of actions to inline actions

[raimund-schluessler]

This forwards a ref of an action to the respective inline action.

This allows to do something like this now:

<template>
	<NcActions ref="actions">
		<NcActionButton @click="actionDelete" ref="actionbutton">
			<template #icon>
				<Delete :size="20" />
			</template>
			Delete
		</NcActionButton>
	</NcActions>
</template>
<script>
import Delete from 'vue-material-design-icons/Delete'

export default {
	components: {
		Delete,
	},
	methods: {
		actionDelete() {
			alert('Delete')
			console.debug(this.$refs.actions.$refs.actionbutton) // Not defined before this PR.
		},
	},
}
</script>

However, the use case in nextcloud/server#34288 won't work anymore without adjustments, because we use a render function now, and the ref is not available in the context of the current component anymore.

[raimund-schluessler]

/backport to stable6