Update sash visibility based on check of all ViewContainerParts

[colin-grant-work]

What it does

Fixes #10894 by updating sash visibility for all ViewContainerParts when any ViewContainerPart is collapsed or uncollapsed or changes visibility.

How to test

1. In a view container with many parts (e.g. SCM with GitLens installed)
2. Confirm that sashes are visible and can be used to resize parts if you are below the top of the first uncollapsed widget and above the bottom of the last uncollapsed widget.
3. Confirm that sashes are invisible otherwise.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[kaiyue0329]

The changes look great 👍 ! I was able to see

• No sash when all ViewContainerParts are all collapsed
• No sash when there is only one uncollapsed ViewContainerPart in the left panel
• Sashes for all the ViewContainerParts between the firstUncollapsed and the lastUncollapsed

[vince-fugnitto]

I confirmed the bug on master and that the pull-request resolves the issue 👍
I did not notice any regressions, and the code also looks good to me.