Add prefix to contributed view container ids

[tsmaeder]

What it does

Adds the prefix workbench.view.extension. to the id of view containers contributed by plugins. As the id of the view container now contains dots, it can no longer be used as a css selector, so we now generate a sequential id.

Fixes #13249

Contributed on behalf of STMicroelectronics

How to test

I use the tree-view-sample from https://github.com/microsoft/vscode-extension-samples/. The example contributes a view called "Node Dependencies" in a container called "Package Explorer" plus it adds a bunch of views to the standard "Explorer" view container. Make sure you can open/close/reopen the "Package Explorer" view and that all the contributed views are functional, including showing the proper icons.

Follow-ups

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

[rschnekenbu]

That looks good to me.
I also checked that the 'edit' menu is still showing on the node dependencies item in the tree view sample while adding this check to the 'when' clause of the command: "activeViewlet == 'workbench.view.extension.package-explorer' &&". The behavior is now similar to vscode one.