Separate XWikiPreferences from each other

The XWikiPreferences contains critical settings such as the users, groups and access rights as well as non critical information like the theme, color and sidebars.

On upgrading the wiki the upgrade manager tries to merge conflicting files. E.g. small changes on the view of the wiki could lead to change on the XWikiPreferences, and the upgrade manager tries to combine the new factory settings with the users preferences. The merged version often affects the critical settings mentioned above. So the user has the choice to reset those critical settings after the merge or to regain the really new changes after keeping the old XWikiPreferences.

Thus those settings should be separated into finer granularity. This has many advantages. During an upgrade there is no interference between smaller preference groups. Further it is easier for the upgrade manager to merge smaller preference groups. Things become more safe and simple.