XWIKI-9341: Menu improvements when Workspaces activated.

1 - Rename "Main" menu to "Home".
2 - The new menu is:

* Home
  * Administer Main Wiki (could be Administrer Home or Administer System)
  * Wiki Directory
* Wiki
  * Watch wiki
  * Administer Wiki
  * Document Index

3 - The home menu is displayed if:
 - Workspace Manager App is installed
 - There is at least 2 (sub)wikis (workspace-template which is hidden + a real wiki) - so it is not displayed if the user wants to use a single wiki.
 - The user is not a local user (a guest in workspace or in the main wiki is not considered as a local user).