Rework how file selection is shown

[marcelgerber]

This PR includes two changes:

• The dark background element behind the currently selected file in both the file tree and Working Files has his width now handled by CSS (turns out that's a whole lot easier than to have React update it on every change)
• Upon resizing the panel, the tree is rendered without delay, that is, the "selection extension" that covers the dark scrollbars is always in the right place. Fixes SidebarView resize shows ghost of File List selection in editor pane #11341.

[ficristo]

LGTM after testing briefly.
Maybe @petetnt could take a look too?

/cc @humphd because it seems to me he has fixed the same problem in a different way so he is aware of this change.

[zaggino]

Reviewed & tested. Fixes referenced issue so merging.