User profile "Skin" field suggests random pages instead of skins

The "Skin" field in a user profile suggests the wrong pages:

• It suggests any visible page of the wiki, not only skins.
• As a result, users can pick something that is not a skin (for example a color theme such as
  FlamingoThemes.Darkly, AnnotationCode, or other user profiles) and nothing happens: the page looks the same and there is no error.

Present since: https://jira.xwiki.org/browse/XWIKI-15452

Steps to reproduce

1. Open a user profile in the Objects editor and expand the XWiki.XWikiUsers object.
2. Start typing in the "Skin" property.
3. Look at the suggestions.

Expected: the list shows the wiki's skins, including XWiki.DefaultSkin.

Actual: XWiki.DefaultSkin is missing. Ordinary pages are suggested. If a hidden color theme such as FlamingoThemes.Darkly is unhidden, it is suggested too, even though it is not a skin. Selecting it has no visible effect.