Load the Thumbnail in the Background

[ismailsunni]

Feature description

Currently, we have around ~160 resources. It will take some time to load them all for the first time. I guess, it's time to put the thumbnail downloader in the background using QgsTask and show it after we download it.

• Call the download thumbnail in the background with QgsTask
• If the thumbnail is not ready, use a default thumbnail
• After the download is finished, we should update the thumbnail
• When a thumbnail is clicked and shown in the preview, it should directly download the thumbnail (without going to the QgsTask) to have smooth user experience

Additional context

No response