fixes #76419

The panel currently exposes a `disposables` protected member. In order to catch possible leaks of disposables, we should extend `Disposable` instead

* Added `cachedScrollTop` to SCM panel

* Moved `cachedScrollTop` usage to `layoutBody()`

Part of #74250

- Extend `Disposable` in classes where it makes sense
- Use `DisposableStore` for lists of disposables
- Make `combinedDisposable` take arguments instead of an array (so that you can't pass in an array and then modify the array after the fact)

Fixes #74242

Our current usage of dispoable arrays can leak disposables (see #74242 for details). This change introduces a new class called `DisposableStore` that can be used mostly as a drop in replacement for an array of disposables but will not leak disposbles if it is disposed

`DisposableStore` was extracted from the existing `Dispoable` class, which already implements this pattern. However `Disposable` is intended to be used as a base class while `DispoableStore` is a value class.

In addition, this change hides the `toDispose` / `_toDipose` members of `Disposable` as direct write access to the disposable array also allows for leaks (and breaks encapsulation)

#70020

related to #70065

This reverts commit 969bc0d7.

related to #70065

fixes #69688