Dispose explorer view welcome correctly

[msujew]

What it does

Closes #11283

As we call registerViewWelcome each time we recreate the FileNavigatorWidget, we need to clean up the disposable that we create, once the widget is disposed.

Also localizes the view welcome text.

How to test

1. Close your workspace
2. Close the file explorer (using the Close context menu entry)
3. Open the file explorer
4. Assert that only a single view welcome entry is displayed

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

[vince-fugnitto]

The changes work well for me 👍

However, instead of adjusting the caller, should we not update the registration of view welcome so we do not register duplicate content? (I don't see a reason as to why we would want duplicates). I also believe vscode uses a Set for this reason.

[msujew]

@vince-fugnitto Thanks for the quick review. I agree, registering multiple elements with the same content should be prevented. I've created a commit for that.

But we should dispose of the created view welcome here anyway, since it creates entries in the viewClauseContexts, which should be deleted when the widget is disposed.

[vince-fugnitto]

LGTM 👍 I confirm that it fixes the issue reported in #11283. The only difference in behavior is that vscode does not only check for content as duplicates but I think it should be fine.