Add support for Nested Pages in Link Generator

Now that XWIKI-12920 has been implemented, all wiki links such as [[whatever]] lead to whatever.WebHome and thus the default link label generator displays "WebHome" which is not nice at all for users.

We need to modify the link label generator:

• Have the %p configuration shows the page name OR the last space name if the page name if WebHome.
• (bonus) Have an option for displaying nested spaces since %s only displays the last space name.
• (bonus) Also have an option to force always displaying the page name (in case the user wants to display WebHome for example)