Label updates are not refreshed in mini browser props

[westbury]

Description

This is a follow-on issue from #5884 as per @akosyakov's comment #5884 (review)

Props of the mini browser are not updated if they are computed from the label provider. W probably should handle it here: https://github.com/westbury/theia/blob/59b4ad001e6f57cb823dab56a23cdb32a4c50868/packages/mini-browser/src/browser/mini-browser-open-handler.ts#L126 It could be a bit tricky, since we should not do it when name and iconClass provided. It would be good to have a follow-up issue at least.

Reproduction Steps

With #5884 included, run the 'Examples: Toggle Dynamically-Changing Labels' command. View properties of the mini browser (in case where they are not provided so label provider is used). Use URI that contains 'test' in it. Check that a increasing numeric suffix is prepended.