Extend JSON QStandardPaths to support multiple paths per location

Extend JSON QStandardPaths to support multiple paths per location

Allow a location to be specified as an array of paths.
    LOCATION: ["first", "second"]

The first value is selected for writableLocation().

Define the first entry as an empty string for no writable path.
    LOCATION: ["", "second"]

A single path may be expressed as a string.
    LOCATION: "only"

Change-Id: I897cf40a039ad7cb680bdf643bfa78020e8eb1cb
Reviewed-by: Lincoln Ramsay <lincoln.ramsay@nokia.com>
Reviewed-by: David Faure <faure@kde.org>