fix(files): make sure the FilesList is always mounted

[skjnldsv]

A bit of a change on the logic, this will help #53739

1. Instead of rendering empty contents and error messages outside the FilesList, thus unmounting it, we inject the content as a slot
2. We provide a dummy root Folder in case we don't have a cached root one
3. We remove the duplicate FilesListHeader logic
4. We ensure FilesListHeaders are always mounted thus avoiding issues when destroying/mounting it again, and we improve the overall performances
5. Adds a redirect after page is done loading in case the url doesn't match a valid View. This is a weird edge case, but I figured it would be clean to have. E.G, try accessing https://dev.domain.com/apps/files/FAKEVIEW

[susnux]

Rebased onto new default view feature

[szaimen]

🐘

[skjnldsv]

@susnux something needs a fix on the scrolling tests 🤔
I'm still investigating :)