Allow user to select skin/theme in their profile settings UI

It would be convenient to allow users to select their preferred skin (perhaps from selected collection of themes/skins).

There is possibility to edit skin for a user as described in https://www.xwiki.org/xwiki/bin/view/Documentation/AdminGuide/Skins#HChangingtheSkinforauser

I propose to add this field to Preferences tab in user profile settings. At first it could be just text field.