Display feeds as tree in RSS downloader

[Pioooooo]

This PR changes the RSS downloader dialog to use a tree for its feed list.

It is annoying if there are a lot of RSS feeds to be selected. By using a tree display it reduces this one-by-one selection effort and is more organized and consistent with the RSS reader feed list.

Closes #16802, #1805.

Currently not adding icons for the tree as the previous feed list didn't, but maybe it is better to display one to indicate one is a folder or feed?
Also for the root folder is there need for a name?

[glassez]

Also for the root folder is there need for a name?

It would be nice to see something different from an empty space. Maybe the *nix-like name of the root folder / would do?

maybe it is better to display one to indicate one is a folder or feed?

👍

[Pioooooo]

Added icons and root path. I think it is okay not to show RSS isLoading or hasError icon, as those are mostly temporary stuff and the site icon helps find the feed?

[glassez]

I think it is okay not to show RSS isLoading or hasError icon

Exactly. It would be too superfluous in this place.

[Pioooooo]

Should I do a rebase or what to remove the trailing spaces?

[glassez]

Should I do a rebase or what to remove the trailing spaces?

It's better to rebase.

[glassez]

@Pioooooo
What is the status of this PR? Do you intend to finish it? It's still marked as a Draft.

[Pioooooo]

@Pioooooo What is the status of this PR? Do you intend to finish it? It's still marked as a Draft.

Sorry, I am not familiar with GitHub pull requests and it is now ready for review I guess.

[luzpaz]

@Pioooooo can you rebase ?

[Pioooooo]

@luzpaz I think it's done now.

[luzpaz]

I haven't tested but this looks cleaner. Anyone who uses RSS often care to test and weigh-in ?