Item14530: Add new config "RootDir" to unify configuartions of file system paths

[bixycler]

I propose a new config parameter "RootDir" to have all other dirs set relative
to this RootDir. Their relativity is only set up at bootstrap config, and the
admin can later freely change them to whatever he/she wants.

So the default settings should be: XxxDir = $Foswiki::cfg{RootDir}/xxx, and
RootDir is auto-detected (at bootstrap) to be the root dir of the website.

Known issue:

• On page configure > General settings > File system paths, the subdirs are sill shown in absolute path with a button "Reset to default value $Foswiki::cfg{RootDir}/xxx", but the actually values to be saved are in form "$Foswiki::cfg{RootDir}/xxx". To show the correct form on config page, the Query.pm/getspec() should be modified a little bit.

[gac410]

I've merged this into a new Item branch off of our master branch. Item14530. We can test it from there and review the changes.

[bixycler]

@gac410, thank you for creating branch Item14530 and merging my request.