Use more specifc icons

[sumedha-swamy]

• Main icon for the extension on the left panel: Can the main icon be a bucket rather than a cloud to help users easily understand that this extension is about browsing S3 buckets ?
• Icons for buckets and folders: In the main browser panel, can we distinguish the icons between buckets and folders ? As of now, they both use the folder icon. Operations that can be performed on buckets are different from operations that can be performed on folders. Therefore, distinguishing between them is important.
• Icon for files: For files that dont have an extension, the folder icon is used. Use a file icon instead so that the user understands that they must download that file to view it rather than clicking it expecting the folder to open up.

[DenisaCG]

Re-opening this, as only the first point was addressed in the linked PR so far.