Sub pages can be refreshed

[rodja]

Motivation

If just some data changed (like setting another translation or similar), it is tricky to refresh ui.sub_pages. They will not react to ui.navigate_to(ui.context.client.sub_pages_router.current_path) because the path does not change and hence the re-evaluation is skipped.

Implementation

This PR introduces refresh methods for ui.sub_pages and ui.context.client.sub_pages_router. The first only refreshes itself and all nested sub pages elements. And the latter one refreshes all currently shown sub pages elements.

Progress

• I chose a meaningful title that completes the sentence: "If applied, this PR will..."
• The implementation is complete.
• Pytests have been added.
• Documentation has been added.
• Update "Added in version ..."