Allow delete subtopics(bulk operations) on filtered subtopics

[tablatronix]

Bulk operations like delete subtopics(retained), applies to all subtopics.

It is not possible to filter topics or only perform this operation on on topics shown from search.

Pick a topic, add a search to only show the topics you want to delete, note that the delete button shows the same total, selecting all and bulk work neither, only previews the last item.

#69

[ilgrank]

The project is not actively developed anymore, and the original dev is seeking for help in maintaining it, see: https://github.com/thomasnordquist/MQTT-Explorer/issues/885.

Should you be able to help, or know someone who could, or you're just willing to spread the word about this... any help would be very appreciated!