…#42549)

Whenever right-click was used on the editor, the pane's tab buttons
would flicker, which was confirmed to happen because of the following
check:

```
self.focus_handle.contains_focused(window, cx)
    || self
        .active_item()
        .is_some_and(|item| {
            item.item_focus_handle(cx).contains_focused(window, cx)
        })
```

This check was returning `false` right after right-clicking but
returning `true` right after. When digging into it a little bit more,
this appears to be happening because the editor's `MouseContextMenu`
relies on `ContextMenu` which is rendered in a deferred fashion but
`MouseContextMenu` updates the window's focus to it instantaneously.

Since the `ContextMenu` is rendered in a deferred fashion, its focus
handle is not yet a descendant of the editor (pane's active item) focus
handle, so the `contains_focused(window, cx)` call would return `false`,
with it returning `true` after the menu was rendered.

This commit updates the `MouseContextMenu::new` function to leverage
`cx.on_next_frame` and ensure that the focus is only moved to the
`ContextMenu` 2 frames later, ensuring that by the time the focus is
moved, the `ContextMenu`'s focus handle is a descendant of the editor's.

Closes zed-industries#41771 

Release Notes:

- Fixed pane's tab buttons flickering when using right-click on the
editor