Widget Visibility: Add Parent Options for Categories.

Looks like this may have been implemented in the past (PR #2582, #1153) but doesn't appear to be in place any more.

Currently when assigning a widget to be visible in a specific category there is no option to apply to children, and for sites hierarchical sites this can make the process of setting visibility very time consuming.

Current settings:

If we could add a check box there to apply settings to child pages, categories, etc it would be of benefit to many users.

The issue names no files, tests, or entry points. Start by tracing the widget visibility settings for category assignments in the Jetpack WordPress plugin, then review the earlier PRs #2582 and #1153. Done means the settings provide a child-application option that works for the stated hierarchical page and category cases.

Written by the indexing model from the issue text.