ui: Implement graceful autohiding for scrollbars (#39225)

How it looks:


https://github.com/user-attachments/assets/9a355807-5461-4e8d-b7a8-9efb98cea67a

Idea behind this is to reduce flickering in areas where nothing is
happening - whenever these hide, the user is specifically not
interacting with them, hence it can be distracting to have something
flicker in the side of your eye. This PR tackles this.


Release Notes:

- Added graceful autohiding to scrollbars outside of the editor

---------

Co-authored-by: Danilo Leal <67129314+danilo-leal@users.noreply.github.com>

crates/editor/src/signature_help.rs

@@ -389,7 +389,7 @@ impl SignatureHelpPopover {
                             )
                     }),
             )
-            .vertical_scrollbar(window, cx);
+            .vertical_scrollbar_for(self.scroll_handle.clone(), window, cx);
 
         let controls = if self.signatures.len() > 1 {
             let prev_button = IconButton::new("signature_help_prev", IconName::ChevronUp)